ETA: I saw this thread about MPH and thought I'd post the below. I believe the boxes are the same size etc. If you do need to buy a head and put it in the box, make sure you stuff the inside!
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I had to post back and show my Potato Head Parts...I went to the Toon Town shop at the MK since we didn't go to DTD, I wish I had a better selection of accessories (pretzel, soda) though.
I got about 40 pieces. The Mary Poppins umbrella did not fit but a CM told me it didn't have to go into the box. I paid an extra $2.45 for it. Fairy wings did fit.
I did not get a head, so these were just pieces in a VERY stuffed box, it barely closed and was bulging but the CM said it was fine and in fact asked me if another CM had help me stuff it (nope, just advice from the internet).
Note the bottoms of the mickey feet are very spacious and I stuffed some pieces inside before placing them in the box (name tag and earrings). I have multiple items since I just crammed whatever would fit.
ANOTHER TIP:
If you buy this early on in your trip, each night when the kids go to sleep, make up a new potato head configuration and leave it out for them to find in the AM. You can also maybe play a game where you hide the MPH in different parts of your hotel room each night and the kids can find the surprise in the AM.
Have fun!