These sorts of things are difficult to answer, because: a) What you think is acceptable quality or file size may not be to someone else. b) In general, the smaller the file size, the lower the quality - you can't easily have both. c) In general, the faster the export, the lower the quality - so adding this requirement makes it even harder. I would suggest that you try the public beta version of MixEmergency 3.3.0. The export system in this version should be much faster than in all others. I would suggest exporting some small sections of a mix at different bitrates and figuring out which is acceptable to you in terms of speed, quality, and file size.
