If you look at the format of a hex file, each line starts with the number of bytes that are in the line, with a maximum of 32 bytes, there is a blank : and then the address of the first instruction. The line finishes with a checksum.

VERY VERY Roughly, the number of lines x 32 is roughly the number of bytes of code. You should have a rough idea of how many one, two and three byte instructions you use in your code to weight the numbers.

For a very dirty answer, I just halve the number of bytes in the whole hx file. It isn't going to be an EPROM size wrong !

Steve