Probably some "hide cd-r media" type option enabled... do you use CloneCD, or Alcohol?
It is written in Plextools' FAQ:
What can be the reason for PlexTools to display the error message â€˜Source disc too largeâ€™ after the start of a writing process?
[I]Answer:[/i] This warning message may be caused by a CD-recording software that has a 'Hide CD-R' option (e.g. CloneCD, Alcohol 120%). Please deactivate this option in the recording software or remove the program.
Note: this function can remain active even after closing the program.
Hope this helps,