it's usually important to replace it with the proper size, although you can sometimes use a SLIGHTLY larger or smaller size if you can't get the proper size.
Let's explain why with two scenarios:
1) Let's say you need to replace the brightness pot, and it was originally 100k-ohms. All you have is a 50k-ohms. You may need 80k-ohms of resistance to get the brightness completely down. With a 50k-ohm pot, you'll never be able to reach the level you need, so your brightness may always be too high.
2) Let's say you need to replace your horizontal hold pot, and it was originally a 5k-ohm pot, but all you have is a 100k-ohm pot. Your horizontal hold may only lock in between 1.8k-ohms and 2.1k-ohm. For a 5k-ohm pot, this range is close to the center of it's position, and you will be able to turn it a little in that range without it losing the hold. With a 100k-ohm pot, that same 1.8k-2.1k range will be a super-tiny spot in the complete range of the pot, and will be very difficult to hit when you try to adjust your picture....