What size wheel do you need for a 25t sprocket?

I'm assuming you're talking about a BMX here, which use the Words a bit different from the rest of the cycling world.

So it's the size/tooth count of the freewheel that you're asking about.

And then it's just as much about what you want as what you need.

When you change the sprocket/freewheel size you change the gear ratio.

A big sprocket and a small freewheel lets you go fast, but you'll have to push harder.

A bigger freewheel will cost you some top speed, but will make it easier to get the bike moving.

A very common combo is 25t front and a 9t rear.