I was a (proud!) speaker at ScienceOnline London 2008 -- which would not have happened without my readers' kind and generous support! However, when I was in London, I was given a t-shirt as part of the goody bag that was, to put it politely, a tad small.
Okay, it was smaller than a "tad small": even though it was a "medium" size -- yes, I do wear medium and have lots of room left over, too -- this t-shirt was so small that my not-huge-at-all boobs emphasized certain portions of the "ScienceOnline London" phrase. It scared me so much that the t-shirt and I parted company before I relocated to Germany. SADLY! Sooooooo ... my suggestion is PLEASE make those t-shirts larger! If you're going to provide "medium" t-shirts, then make sure they're really a medium size! Make those t-shirts a wearable size! Make them a size that doesn't make me look like I am a scientist with boobs squeezed into that tiny shirt! (Because we all know that there's no worse fate than being a scientist who is a grrl).
It sounds like ScienceOnline London = Hooters! Now on with the Sulidae and Paridae jokes!
I even get "XL" shirts that make *me* look like I have boobs. Maybe I should moonlight as the bearded lady - the bearded lady with hairy arms even.