What are the main sub-areas of theoretical computer science?
Also I wonder if 'size' is the right metric. that tends to vary as subfields come in and out of fashion. For example, a visitor from the 1980s would be very puzzled as to where all the parallel algorithms disappeared to. Someone attending SoCG in the last five years would think that we should call it the Symposium on Computational Topology :). 

Maybe the best thing to do is to let the student browse proceedings from theory conferences to get a sense of what's in vogue RIGHT NOW, and emphasize that this answer will change over time.