I have 2 of the Large MT Birks .. one in Black (Gold HW) and the other Dark Brown (also Gold HW). The Large should be labeled XL .. 'cos it's huge!!! ... but it's PERFECT for me. I've been using the the bag for (at least) a month .. everyday .. carrying my stuff to/from work and I must say, it's a work-horse! I love that about it; I don't have to carry a purse plus a tote to carry all my files/books, etc.
I originally ordered the large in the Dark Green, but it was so dark that frankly .. it looked Black and I didn't want another Black bag. So, then I decided to go with a Medium Dark Blue. I would say that the Medium size is equivalent to the 30-35cm Birkin .. way too small for me, but probably perfect for most.
Last but not least, as I was rummaging around my house this last weekend, 'lo and behold .. I found an old bag that I had picked up on one of my trips to Italy .. and was shocked to see that the tag said "Marco Tagliaferri"!!! Not sure what year it's from (I would guess from '05-'07), but it's definitely the small .. which is QUITE small .. I would say, equivalent to the little Proenza Schouler PS1 clutch; probably not a great everyday bag. Interestingly enough, the leather on the small bag is much thicker (which is what I remembered) .. I'll post pics this weekend.