How can I tell if my snake is overweight?

[NewSAHSnakeMom]

So we've had Blaze since July, he's about 2' long and at points as big around as my thumb. He's a sinaloan milk snake. He eats 3 small fuzzies every 7 days. I am guessing he's about a year old. The pet store wasn't very knowledgeable but my 5 yr old had to have him. (Her bday present lol) I will post a pic that's about a week old but he's in shed again so I can't take a new one. He's shed every month since we got him. I thought they grew longer then fatter but he's bulked up a lot in the six months we had him. The pet store was only feeding him pinkies and everyone said fuzzies we needed. He'll eat 3 most of the time, only two near shed. He eats so easily I worry about over feeding him. We do try to wait until he's hunting to feed him but it's usually every 7 days. I just don't want to make him unhealthy and fat! I wouldn't worry but he's bulked up faster than I thought. And also how soon should I move him to a bigger terrarium? The one we have is 24x18x18 and he already can't completely stretch out, not that he does it often anyway. Thanks y'all in advance.