Hmm, I've never had a problem with that at all. I just pick the default target of DVD5 and it all works out nice. Done scores of Mpeg2 burns that way.
I was curious, so checked the .mp4's I'd been making, to see how similar they were in size. I'm lately just defaulting everything to the same size, not really caring what the content is, so that I won't have to hard a time (if I ever decide to burn them to disk) finding files that all add up to 4.7 billion bytes. I was frustrating myself earlier, never being able to put a compilation together that didn't either total 4.8 or 3.8 billion bytes. Seemed like a waste. So I figure if Recode reckons one DVD5 is 4,459 MB (it's default value), I'll call it 4455 just to be safe, and tell it to create .mp4's of 1,485 MB so that I can fit any three files neatly on one DVD5; and at that file size the .mp4 quality looks great enough to me when played through my television for any movie under 2 hours.
(BTW - I've been wondering if anyone can tell me how many hours of .mp4 they think they can squeeze on a DVD5 and still comfortably keep what they feel is "DVD quality" - but that is another thread).
In any case, I checked the files sizes of all the files I asked Nero to make 1,485 MB and they all are accurate to about 5 million bytes of each other, or about 1,549,486,433 bytes each. Which puts my compilation sizes at about 4.65 billion bytes, right on target.