Hi there,
Currently, MaterialUI doesn't really support individually resizing many elements. Though it is on our feature list, I couldn't really give you an estimate on when we'd have that done, sorry.
The best workaround I would suggest (which may or may not work, depending on what other UI elements you use from other assets, would be to change the scale on Unity's CanvasScaler component - although this will scale everything under in that Canvas.
~ Declan.