I haven't seen a theme written in the invite, except for costume parties. Usually the invites are a part of the theme. For example, a Hello Kitty Party would have Hello Kitty on the invite. I don't think it's a big deal to include this information in writing, though.
I also have never seen sizes in the invite, but I have been TOLD (without asking) the child's correct size when calling to RSVP. I think it's super rude to put this info in the invite or to offer it up without being prompted. However, I know that people will shop without getting gift reciepts/using common sense. I understand why sizes would be included, but I still think it's super tacky.