inner tubes will be marked up with the range that they cover so if you take it out and have a look then it will tell you.
the ranges do vary a lot so its impossible for anyone to give you a definite answer without knowing what you have.
usual sizes are 700 x 18-23 and 700 x 25-32 but you may have a size that covers both 23 and 25.
as for using the "wrong size", well im not really qualified to comment