I don't know whether Easy 'reserves' buffer space, but Windows has some rather vague defaults about how much temp file space it will allow, based on the remaining free space on the system drive.
Some programs cite "requirements" that include the space to put the download plus the space to unzip something plus the space to make the installation - or something like that; but it sounds as though you're talking about the actual size of the transfer.
The "Easy CD 5 Platinum" install CD I have shows 236,840,741 Bytes for a DOS Dir/s, and there's a separate CD for the "bonus offers," so the size you saw may be good if you're getting the whole thing. The install CD would need to include all the drivers and dlls and such for all compatible systems. The "requirements" on the box say you need 200 MB free space, and at least 80 MB "must be on the system drive" for installation. (That would be after you've moved your download to a CD?)
I don't usually mess with them, but there are "download managers" that will let you do partial downloads and then resume later. This sounds like a good time to get one? I don't recall where I saw the last recommendation for one, though.