Hi OP. So very small colleges are Peterhouse, Corpus Christi and Trinity Hall (all central).
There are a few colleges with more er, 'modern' architecture - Robinson, Fitzwilliam, Murray Edwards (all female) and Churchill. This may or may not be a factor.
Be aware that not all central colleges will accommodate everyone on site - eg. many students at Gonville and Caius live near the Sidgewick site.
For MML she would be in the Sidgewick site, su G&C might be good? Otherwise, Queens or Selwyn would be nearest (or Newnham if she's interested in an all-female college). Also, Pembroke, Peterhouse, St Catherine's or Corpus are that side of town. But nowhere is far, so any college really.
There is a large college a bit out of central Cambridge called Homerton which is known for being quite forward-thinking and more diverse than most colleges. Also accommodation on site for all three years.
A little further out but the other side of town is Girton.
Other large colleges are Trinity, St Johns and Jesus.
Emanuel, Christ's and Downing are lovely. To be honest, all the colleges are! You can't go wrong. Even the 'modern' colleges have beautiful grounds to distract!
You will be able to see more than three in a day.
Hsve a look at the admissions stats per college (interactive graphs) to get a sense of intake numbers / competitiveness for her subject per college (as this can vary year to year). But good news is, MML is about 40% acceptance rate overall.