Project files for 3d printing?

[githubforced2fabad]

I would like to create a parameterized version of the adept. Input ball size, and the whole thing scales to fit the ball. Of course some things will need to remain constantly sized such as the the slots for the rollers, the hole for the sensor, the hole for the usb-c port, holes for the screws, and the space for the computer chip. In addition the buttons will have to extend further to reach the static location of the buttons on the computer chip.

My reasoning is that I would like to have to adept but with a larger ball.

Anyway, I am totally new to 3d printing design. I was successfully able to separate the 3 parts (top, bottom, and what I'm calling the cradle (the part with the hole for the sensor)). It seems like the parts are rather blob-like as in there is no properties on them to change (I'm using free cad). From my reading online, I would need the original project files to do this kind of work (as opposed to the STL files).

Any advice would be much appreciated.