Your inseam and the stand over of the bike are about the least important part of fit. Use this fit calculator. It will give you a ballpark idea of what size bike you should be on.

Bike Fit Calculator | Find Your Bike Size | Competitive Cyclist


Of course if you are new and looking for a bike the best bet is visit a LBS and let them fit you. Getting the right fit is personal because people have different proportions. Some have long legs, some have long torso, some have long femurs, some have long wrists and everything can effect fit. I'm 5'11 and I ride a 54 quite comfortably but many people my height ride 56 or 57.

But to answer your question, for me a bike is too big when I feel too stretched out. If you can't reach the brakes/shifters comfortably it's too big.