This test message in Gmail:
is correctly found by a search for "Length is 7403":
but incorrectly not found by a search for "Length is 7418"
The message body message is 85,075 bytes long and the part up to and including "Length is 7403" is 7,934 bytes (the size having been inflated by emailing since being originally 7403 bytes in the source file).
Any idea what's going on? I really hope Gmail search doesn't fail past 8K bytes on all long messages!