I made my virtual buttons invisible and put the graphics for it on the background image.
I only use 2 virtual buttons so far on my board game.
One is for the help (?) and the other the exit (x) with the (x) serving a double as the exit for the help screen too.
I put in a simple y/n confirmation for the exit for the app itself.
The size didn't matter much for those buttons since they are at the top of the screen anyway.
I made the diameter the same as the width of the graphic then made sure the bottom was right.
So, if you click just above it then it registers the click, but that is okay for what I am using them for.
If you use a round graphic for the button then it works much better than using squares or rectangles.
Another thought might be to use multiple buttons for the rectangle. (see attached image)
I haven't tried this method yet, but it should work.
The only thing to keep in mind is the 12 button limit on the screen at the same time.