OK, the way I do it is right click on the ISO, pick "Add to Archive...", set the compression to maximum, then right under that is an option that says "split into volumes, bytes". If you type in 10M there, it will make 10MB parts. If you type in 5M, it will make 5MB parts. You get the idea. Pick a chunk size, then click OK and it will make a lot of part files.