I knew there must be a limit - there's only 2 bytes for the number of entries, so logically there's a 65k limit on entries. If I'm right about the one byte for the file size, then there's also a 64k limit on the that (hmm - perhaps there's only 2 bytes for the padding offset then - there doesn't seem a lot of point in having 3 bytes if the maximum will fit in 2).

I'll have a play around with generating a very large dictionary and see whether I can hit the upper limits (and whether they help tell me anything about the format).