Schwarzchild Radius
A few calculations to indicate the variation of the Schwartzschild radius with the mass of the object. For these calculations, we'll use the simple formula derived from the escape velocity of light - but remember, this formula is only good for a spherical non-rotating black hole, so that's what we are talking about here.
First, let's define some constants.
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The formula for the Schwarzschild radius for a spherical non-rotating black hole is:
sr = 
So let's apply this to a few masses and see what we get.
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So we see that the Schwarzschild radius of the Sun is 2.953 kilometers, and that of Earth is 8.871 millimeters. So if the Sun and Earth were small enough to have their entire mass concentrated in spheres of those sizes (or less), they would be black holes.
Now let's see how the Schwarzschild radius varies with mass.
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So we see that the relationship between mass and the Schwarzschild radius is linear. In fact, for astronomical calculations, since the mass of the Sun is a handy number to use, we can simply use that:
Schwartzschild Radius per Solar Mass = 2953.43 meters
If we have a black hole of 10 solar masses, the SR would be 10 × 2953.43 = 29534.3 meters.
If we have a black hole of 1 million solar masses, the SR would be 
× 2953.43 meters, and so on.
This leads to an interesting conclusion. For a spherical object, mass is proportional to the cube of the radius (since volume of a sphere is 
π 
). Doubling the mass of a spherical object only increases the radius a little bit (radius increases 
or 1.26 times), but doubling the mass doubles the SR. Therefore, by increasing the radius, the mass increases at an exponential rate, while SR only increases linearly. So mass is increasing way faster than SR, which means we could use a less dense substance to produce the same SR.
In fact, we can take this idea to absurdity. Imagine a huge spherical mass of water in space. It has the density of water on Earth (1000 
), because for some reason it hasn't collapsed under its own gravity. This is a really, really huge mass of water, say 150 million solar masses. Then:
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The radius of this massive sphere of water would be about 4.14 × 
meters. What would the Schwarzschild radius of this mass of water be?
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So the SR is 4.4 × 
meters, which is larger than the radius of the sphere of water. This means that the mass of water is completely contained within its Schwarzschild radius, and thus it would act as a black hole, with an event horizon outside it!
Of course, this is not totally realistic, because a mass of water that large couldn't exist at such a low density. It would collapse under its own gravity into a real black hole.
But it goes to show that very large black holes, such as those at the center of galaxies (which mass millions of solar masses) have really large event horizons. The black holes themselves may be tiny objects, but the region of space which marks the boundary of no-return around them is pretty darn huge.
Largest Black Hole
The largest black hole discovered so far is the object at the heart of the quasar known as OJ287, which is estimated to have a mass of 18 billion suns. The Schwarzschild radius for this object would be:
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The radius of the event horizon is therefore 53 trillion meters, or over 7 times the radius of the entire solar system (counting aphelion for Pluto). This is a truly massive object, too bad it's so far from us - about 3.5 billion light years away from Earth.
Time Dilation
Gravity fields slow down time, as per general relativity. Assume you have 2 observers, A and B. One of them, A, is approaching the black hole. The other, B, is watching him from a safe distance away. We are assuming that A is close enough to the black hole to be strongly affected by its gravity field, while B is far enough away to not be strongly affected.
For B, it looks like A slows down as he approaches the black hole. The closer A gets to the black hole, the slower he seems to go. In fact, B would never see A entering the event horizon, because to him it appears that time slows down to a stop at the event horizon. He would see A forever approaching the event horizon, but never quite getting there.
Of course, for A, the situation would be different. He'd get caught in the gravity and zoom right in towards the black hole, like everything else does. He'd go right through the event horizon and eventually become one with the singularity in the middle. He'd be unaware of it, since tidal forces would have torn him into atoms along the way.
We can use the Schwarzschild radius to model this time dilation effect seen by observer B.
=
where 
is the elapsed time for observer A, who entered the gravitational field, t is the elapsed time for observer B far away from the black hole, 
is the Schwarzschild radius of the black hole, and r is the distance of the observer A from the center of the black hole (A is the guy entering the gravitational field).
Let's assume a stellar mass black hole, with a mass of 6 suns.
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So while a whole second has passed for B, the guy who's far away, only 0.23 seconds have passed for A, the guy who's 1 km from the event horizon. In fact, A appears to have slowed down to B. The closer A gets to the black hole, the slower he seems to move to B.
Let's see what it looks like to B when A is really close to the event horizon - say 1 meter away from it.
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When A is only 1 meter from the event horizon, for each whole second to B, only 0.0075 seconds have passed for A. At this ratio, when 133 days have passed for B, only 1 day has passed for A.
Let's try one more time, this time with a really tiny radius.
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At 1 millimeter from the event horizon, the ratio is down to 1 : 0.00024 between B and A. That's like 11.5 years passing for B while only a day passes for A.
From this we can tell that the time dilation is quite weak at what we would consider tiny distances (1 kilometer) by astronomical standards. Only when you get really really close, like a meter or millimeter away does most of the time dilation occur. So what observer B would see is that A seems to zip towards the black hole very very fast, but when he's within a few meters of it, he seems to slam the brakes and then comes to a stop right at the event horizon.
