fabric.js Resizing a rectangle with zoomeffect

I'm working on this designer for custom signs. http://ift.tt/1FgeGO8 My next task is to resize die gray recteangle using exclusively the NumSteps.

How kan I do this?

And now, the hard part comes: The rectangle schould always use the maximum possible space inside of the canvas (the yellowish background). It's hard to describe so please check out this disigner an play around with the numericSteppers. http://ift.tt/1DLxeXX While the sign can get a measurement of 300 x 150 cm it gets "zoomed out" to fit on the stage.

Woukd love to know how to do this in JavaSchript with fabric.js

Thanks alot