For recording? Best way is VBR with 0 buffer size, then use CRF to determine the quality value.
So to sum it up: turn CBR off, ignore bitrate, set buffer size to 0, then use a CRF value of 16. (CRF will be determining the quality of your file). Lower CRF equals more quality, but also higher file size.
Another nice tip that people don't often know is that you can also use lower CPU presets such as superfast or ultrafast to reduce CPU usage of encoding if you're willing to lower the CRF value a bit more to compensate for the quality loss (though it'll further increase file size), but since you're trying to keep the file size low I'd recommend not doing that and just using veryfast.
If you want to get smaller file sizes, increase the CRF value. Eventually you'll find the value that you like the best where you get just the right quality with just the right file size.
