25% = 1/4 of the size. Full size would be 4x that original size. If you copied it at 100%, that would be just copying it the same size it is. So you need 4 x 100% (or 400%) to make it the full size.
The easiest way is to take it to a copy machine (supermarket, Staples, etc.). When you do it on the computer, it doesn't always come out right, especially if you do it in increments or if it's larger than the standard 8.5 x 11 paper size.
Isn't math fun? LOL