If file size is important, then a relatively predictable encoding method is to use Average Bitrate and calculate what is needed to fit the file size limitation. For instance, e-mail probably has a 20 or 25 MB limit. The average bitrate can be calculated to create a file that size given the duration of the video. The following thread gives a demonstration of the math. Meanwhile, estimating the file size in quality mode (CRF/QP) has no predictability at all.