I found this on TMPG.com's website. I created my own very simple bitrate calculator (that was off by a little). Anyway: This may help if you're trying the same thing:

If you choose CBR or 2PassVBR, the following equation can be used to calculate the size. (In the equation "2048/2018" stands for bitrate of the system stream.)
File size (Kb) = (Video+Audio) x (2048/2018) x sec/8

E.g. Video is 1150Kbps, audio is 224Kbps, 15 sec MPEG file would be "(1150+224) x (2048/2018) x 15/8" = 2614KB