Yes, that's effectively what I'd like to do: Control the x-y dimensions of a tree map!
So this is a tricky one...the Tree-map (for good reasons) acts a little differently to the other chart types in Tableau, in that Tableau determines the layout of the boxes/squares. It orders them by size, but Tableau determines how the area (from a length/width perspective) is drawn...this is so it ensures a final rectangular filled pane.
One thing that *might* work here would be to create a marimekko chart
While everything needs to be in single column (for each X dimension), you do get the control of X (with the given caveat) and Y lengths.
If this will work or not will depend on your exact situation...here is a link to how to do it
If you can post some sample data, with some information on how you want to control the X & Y, I'm happy to take a look.