You should be able to scale up the machine just by using longer metal rods, and use the standard reprap plastic parts.
How much bigger are you thinking?
You may run into a few problems with the steel rods becoming less sturdy if you make them a lot longer, to overcome this you would need to user thicker rods (and some re-designed reprap parts for them to fit into). If you want to make one a lot bigger, you can always build a standard machine first, then use it to print larger parts.