I personally prefer 13 inches. It's the ideal size, full-sized keyboard fits, everything fits on the screen, and it keeps the weight light. By the way, a 15 inch doesn't necessarily make the font bigger (in fact, it's usually the same size). It just fits more text of the same size horizontally. You can magnify your web browser font on a 13 inch and 15 inch if it's too small. As for battery life, I don't think a 15 inch makes the battery life worse as a 15 inch means more room for a bigger battery to offset the more powerful processor in the 15in MBP that eats more power.