You can tell just by looking at the temp folder.
The ROM file size will always be the same - 63.2 MB (66,322,432 bytes) - , but the
imgfs_raw_data.bin file will show the actual size that you used (I think) and for sure, right click and get properties on the 'dump' dir and that will tell you what you're trying to sqeeze into 63.2mb - although that seems to be a couple mb larger then the actual Rom size.
I'm at 57mb myself, no issues. Don't forget to remove junk you don't want or need (for me, PMSN is 'junk'
) If you're over 62MB you are in the ballpark of being full.