Does somebody know how to make a customized minecraft world with gigantic plains near a see, that is like around 50 of the Y-axis? I've tried to make a world like this, but i just don't know how. Also I've tried to look up some of the customized world options, not that it was much of a help.

The reason that i want a world like this is because i need a nice place to build a city, without having to remove an overload of trees, and without having to care much about the height limit.