GetMinSize is defined as the minimal reasonable size of a specific Ctrl and does not need to be defined for each one. It is not defined for Button (should it be?), as such use is not typical.
AddFrameSize adds the size of Ctrl's frame to the specified size of Ctrl's view. Not much use for Button, but possible solution.
button.AddFrameSize(20, 50) is likely equivalent of Size(20, 50), unless you do some Frame magic with the Button (normally, Button has single NullFrame).