Hi,
Changing the width and height of regular mesh is not possible, you have to scale them instead, unless you have a framework which gives you access to dynamic/interactive meshes ( houdini does for example)
So, have a slider, ranging from 1 to 2 for example, and get its value, then you set the scale ( on axis only) with that value, and that's it.
Where in that process are you getting stuck?
Typically, when I start with a new feature, I work backward, I first have an fsm that control my scale value, and expose the value in the inspector, then I manually change that value and see how it affect my object, then I let a slider control that value.
Bye,
Jean