I don't think there is a ROM (.rom) format, at least not for CD images. If you mean you want to make a CD image out of an actual game CD, then you have to get a special program to do it. People usually take this opportunity to suggest paid programs that are impossible to get or even don't actually let you do it (Nero doesn't actually let you make an iso, only nrg file that emulators don't read).
Anyway, I can suggest a program called UltraISO. It's a little unstable, it's shareware (has a nag screen asking you to pay for it), but has no 100 MB limit like most other programs like this. I don't know of any freeware ISO making programs that actually work and aren't limited to 100 MB.