They're definitely bigger than a squirrel and very much like a marmot. Yeah, groundhogs are probably around the same size. I did a quick search on Google and read that adult groundhogs are 41 cm long and 3.9 kg. They're actually really cute, but scary if they're trying to get at your peanut butter bagel. *g*

This vid might give you a good idea of their size and nature (when food is involved):

https://www.youtube.com/watch?v=gSo-VBxlRCQ