There is a hard limit of 100.000 bytes i also encountered:
build_pc_track_mfm(track, side, image, 100000, tr.number_of_sector, sects, tr.gap3_length);
If you just sampled more data than the 'dsk_dsk' routine can handle (no copy protection assumed), it should be possible to cut the gaps
(automagically or in a disk editor).
Olivier Galibert is the author of the 'dsk_dsk' importer. Kevin Thacker and Barry Rodewald wrote the Amstrad emulation.
You might have luck contacting Olivier Galibert in your own tongue - or just upload the image so folks can have a look.[EDIT: to increase the limit is not a good idea. Upload to Dropbox or a good free file hoster and post the link here]