First of all, I don't know if this is the right section but it seems to be?

I have a Schwinn S-25 mountain bike, and a couple days ago I took the seat off which was attached to the seatpost and lost it (long story) ... now I need to buy a new seat and seatpost. I have bought a seat, but I can't figure out what diameter my bike will take for a seatpost. I have looked on the bike in numerous places but it doesn't say anywhere. Does anyone know for a Schwinn S-25?