The file definitely isn't too big I've comfortably opened files of over 4MB in CR3 with no problems, in both epub and fb2 format.
I have come across some files I've converted from epub to fb2 using calibre that haven't been liked, either not opening or locking up my EZPro. From what I gather the calibre conversion to fb2 isn't the greatest but as it works for the most part I'm not that worried about it.
I would say the file you are trying to read must have a character or such in it that CR3 doesn't like and is killing the conversion to fb2.
Have you tried converting from mobi to fb2 in case the conversion from epub to mobi (I'm assuming you did this) has stripped the offending character(s)?
Last edited by Mnementh; 10-14-2010 at 03:32 PM.