the size of the file is directly related to the bitrate at which you encode, not frame size or fps. So, if you want a 10s clip to be 15 MB (not including sound), divide 15 MB to 10s and you get 1.5MBps bitrate, or 12Mbps. You might get better quality if you encode 2-pass VBR with max bitrate set to what you computed previously.
Anyway, it's usually better to render your comp to some losless format (QT none, or TGA sequence, etc) then compress your clip with Media Encoder. This way you can play with the settings for H.264 until you get an acceptable result, without having to render from AE every time.
thanks for reply marian.
you're absolutely right I should be looking at data rate. also a good idea to render uncompressed first - time permitting.
I wonder if Media Encoder gives better results than After Effects or do they both use the same render engine and encoding?
honestly, I never tried exporting compressed formats like H.264 or MPEG from AE, but you should get the same thing, I don't think Adobe wrote different encoders for both programs. Also, ME can encode AE timelines directly, so it makes sense that it's the same encoding engine.