when i have done this in the past, i've used streambox ripper to decode the realaudio to wav and then encode to mp3 with lame on a vbr setting and let it decide what bitrate is needed according to its psychoacoustic model. you can play around with the target bitrate if you need to balance filesize against sound quality. i usually have found that "-V 4 --vbr-new" is sufficient for me for transcoded realaudio but have stepped up to "-V 2 --vbr-new" in some cases as well, if the original realaudio was particularly low in quality.
lots of info about lame settings at
http://wiki.hydrogenaudio.org/index.php?title=LAME
hope this helps.
eta: generally speaking, the mp3 will be larger in size than your realaudio original. that's the nature of the beast, unfortunately, since if you were to set the mp3 target bitrate at around the same rate as the realaudio original, the mp3 will more than likely sound worse than the original!
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