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Total calories isn’t a good metric for how healthy a food is. You could eat 800 calories of candy or 800 calories of chicken and vegetables, one of which is obviously much healthier than the other. So the answer will depend on how many calories you want to eat in a day, and the ingredients of the pizza!
That really depends on your caloric needs daily. I’d you’re a small female that’s sedentary, that could be 2/3 of your daily calories. For someone like me who’s a 200 pounds, 6’4 long distance rower, that’s too small for me. Track your daily calories and not just one meal.
In general, no it’s not unhealthy to eat that many calories in a sitting. Some people do a form of intermittent fasting called OMAD (one meal a day) where all their calories for the day are eaten typically within an hour and there aren’t any reported health issues that I’m aware of related to eating only once per day. If anyone knows any reputable findings that show negative health effects I’ll gladly change my comment though
Edit: grammar
If you’re not constantly overeating you won’t gain any noticeable weight form overeating once.
Can’t really say much else without knowing how much everything else was and how much you can have, and at that point you’ve already done the maths anyway.