Expand the file system ( Enable RPi to utilize the full size of your SD card).
Turn on the Porta Pi if you have not already.
Exit the front end, and in the terminal type
Selection the first option, then "Finish" (exit the config menu).
This step will auto reboot, and configure things so that the full size of your SD is recognized. Do this before adding tons of roms. Remeber to leave at least 100MB of unused space on your SD card. The OS needs some space to 'function'.