K.T.
I received a first birthday invite for a little boy that wears 2T and I was glad that the size was in the invitation - especially as a fellow parent - I would never consider that rude.
I know what a hassle it is to get clothes the wrong size: For my little girl's baptism (when she was 3 mo, but wearing 6-9mo clothes), she got so many dresses that were too small for her! No gift receipts. I went to Target, Kohls, BabiesRUs, Walmart, etc to find the dresses and exchange for a larger size - It took weeks! I got them all in bigger sizes, except for one which I re-gifted. I did not want it to go to waste. I am sure some people consider that rude, but what is the point of a closet full of clothes your child cannot wear?
I say - put the size on there. If they are parents, they should be happy for the help :-)