They are classified by mass. Some are definitely bigger than others. The one in our Galaxy called Sagittarius A has as much mass as over 4 million if our suns. If it were in our solar system it would be physically as big as the orbit of Venus.

The mass of black holes is typically measured in multiples of our sun, or stellar masses. A supermassive is measured in the millions to billions of our sun, while a “small” black hole is just a few dozen suns big. The small ones, or stellar mass black holes can hang around like stars and over a galaxy but the supermassive ones are the things that create or hold the galaxy together.

“How can time end” is more complicated. Without getting too complicated with it imagine putting a drop of red food coloring in a pool. Imagine it in slow motion. The big bang is the moment the food coloring hits the water. Then the food coloring spreads out but for a while you can clearly see the red. We are at the point in the age of the universe where the red is still visible in the water. At some point the red will spread out so much you can’t see it anymore. Without getting too dramatic that would be called the heat death of the universe. At that point time effectively stops because there is nothing left to change, meaning the red itself carries time with it. This is also called entropy.

So to travel into a black hole would be to fast forward to the point where the universe has become so spread out that atoms and the bits inside that make up the atoms fall apart, effectively ending time. Time itself is a measure of change. Nothing left to change means no more time. So that’s when the black hole ends. It’s a little more complicated than that but if you can understand what I just said you are about 90% of the way to understand everything we currently think we know about black holes.