When you export to mp3 there are these options in the window:
different bitrate modes have different options in the dropdowns.
I don't know enough to tell you what bitrate mode to use for osu though, I haven't put much research into it since I haven't tried to rank anything yet and I just use constant so I don't have to think about it, if I even bother to compress the file to begin with (I dont really upload most of my maps). Personally I cant really hear the difference anyway. plus different songs would be compressed better with different options.
Variable or preset 170-210 would usually give best quality for file size around 192kbps but you cant predict the exact average bitrate or file size, and I don't know if 192 is a hard limit for that rule or not or if something slightly higher like 194 would be fine.
http://spek.cc/ this is helpful for checking average bitrate