Just select the size you want to use and press ok.
Here's a hint for optimizing picture sizes for FGRV using this tool.
Select Advanced then Custom and enter 600 by 600 in the boxes and your pictures will be resized to FGRV's limits. Up until Booker pointed this out, all my other software made the pictures either too big or too small, now they can be just right.
Your pictures will be renamed to the orginal picture name followed by Custom in brackets in the same folder.
e.g. Original.jpg becomes Original(Custom).jpg