I ran out of time before our trip (& didn't have my PM yet) but I planned on making t's for him. I got the measurements from some wonderful DDA friends that had just gotten theirs. I found this website which is for PM lovers.
http://www.palmickey.net/
It has a lot of helpful info on where to buy the t's that will fit him. I did end up buying him a XS t-shirt from Disney Tails (pet store - don't tell Mickey where we got it) while I was at Downtown Disney & will be putting on a nametag iron-on the front of the shirt (the Happiest Celebration design is on the back). The XS fits him very nicely. I'll be ordering t's for our future trips based on the info from the site I posted.
My DD's and DH aren't interested in having matching t-shirts every day so I want to make matching t-shirts for PM and I instead.