You can set the figure Units property to inches and Position to be [xbase ybase xbase+6 ybase+4] with xbase and ybase to be determined by the lower left corner position.
This should not affect the X and Y axis scales, if by that you mean the data range that is displayed.
I wonder, though, whether you should instead be looking at the figure PaperPosition and PaperPositionMode, which have to do with the size that a figure should render at when you use print() or saveas() ?
