Gaining

You know how people put on weight in all different ways? Health issues from fat vary at least that much. So there is no "safe" size.....but also no fixed "do not get bigger than this" point either.

Huge blood sugar rushes can wear down your body, so learn about the glycemic index and how to identify the simple carbs that cause issues and to know what has more fibre that can balance things.

Stay active! Walking or swimming or the like will help keep your heart strong. Strength is important, both for carrying your weight and because working muscles is good for you. Don't be sedentary for hours at a time...get up and move around occasionally.

And the usual rules of good diet still apply. Eat your veggies, watch your salt, minimize trans fats, etc.

None of this is any guarantee of health, but it helps stack the odds your way.

No such thiing as a weight at which health problems develop....some people are 400lbs plus and fine;while some are 200 and have health problems.
Genetics play a large role and looking back at your family...parents and grandparents,will perhaps give you some clues as to risk areas.Smoking is a big riisk too.
Generally,gain reasonably slowly...it gives your body time to adjust and walk,stretch and do light work just to keep you flexible

You are already at a weight giving higher risk for the health problems. You need to have regular monitoring for high sugar, high blood pressure, high cholesterol. Stick to a low fat diet for the cholesterol levels, but the higher sugar level is related to being overweight, not sugar intake so much.