This depends on wether you want the 32-bit or the 64-bit version of Windows 8. If I remember correctly it's about 2 to 2.5GB in size. When installed the 64-bit version uses 20GB of size, and 32-bit version uses 16GB.
I can tell you that the currently available Windows 8 Pro Upgrade ISO is 2,828,864 KB (64bit version) Of just short of 3 gigs. This is what I got here in the U.S. I've had reports the ISO may very slightly in size based on geography. For instance the same ISO in the UK is reported as being 2,764,736 KB, so a few megs different. Haven't gotten any reports so far with respect to the 32 bit version but I would expect it to be a few hundred megs smaller. Hope that helps. Regards Randy