Well it's not really a bug, it's a limitation. mIRC is a 32-bit program written and compiled for 32-bit processors, meaning the largest efficient integer type available for a filesize is 32-bit unsigned (2^32 = 4294967296 bytes = 4GB). Sure, 4GB can easily be handled by today's hard drives, but even so 4GB files are rare, and using DCC is certainly one of the worst possible ways of transferring them over the net that I can think of. I'm not saying mIRC shouldn't extend the filesize it can handle/display, I'm simply pointing out that no-one would ever notice this limit in practice if they were even remotely sensible.
Assuming Khaled does switch it to a 64-bit unsigned integer, I wonder how long it'll be before someone posts saying that their 16+ exabyte file isn't being displayed at the correct size.