The ease of riding and durability of the bike depends on the design and strength of its frame. Frames are current made in steel , aluminum, titanium or carbon fiber. In most cases the price will scale up along that same order as well.
There should be some gap when you stand over the top tube of the frame. It will allow you to easily maneuver and have a safe ride. If you there is too much gap when standing over the bike is too small for you and you would be cramped in other areas like the reach towards the handlebars, seat height, etc.