You may as well ask how long a piece of string is! A vagina is not a fleshlight. There is no standard size. Women, like men, come in different sizes. Some have deep vaginal vaults and some have shallow ones. Some have stretchy vaginas and some don't. And so on. What is too big for one woman will not hit the sides (or depths) with another. This is why women are satisfied with penis sizes ranging from average - enormous and that is why penis size matters more to some women than to others too. For this reason, your question really cannot be answered in any meaningful way.
Does size get better up until it is too big? It depends what you mean by size Girth matters but length is length. My vagina can only take a finite amount of penis length - anything more than 8.5in won't have anywhere to go with me and my preference would be to have somewhat less than the full 8.5in; 7.5 - 8in would be ideal, in terms of unrestrained activity between the sheets. Girth is the thing that makes things truly interesting. For me, the ideal girth is about 6-6.5 inches. I would be game to try more than that but I have no real idea how much would take me into 'too much' territory...