re: How bad is eating a whole large pizza once per week?

Several factors involved, including age of the individual (18 years vs 28 vs 38, different baseline calorie needs), exercise level, type of work (sitting all day vs on feet and mild lifting like waiting tables vs heavy manual labor), and type of pizza.

A large thin crust Hawaiian and a large pan meat lovers are probably 2 to 3 fold difference in calories.

If you're 18 with an active type of job and you get some exercise, you're probably fine if you're doing well diet wise the rest of the week, no matter what pizza you get. If you're twice that age with a desk job with limited exercise