Go to widows, model info and drop down shows a units, suggest you use decimal vs fraction, you can set accuracy ( display accuracy I usually use 3 decimal).
When you first use the stl import in SU there should be a flash screen and it gives options of mm or meters that is where I selected meters. The stl format is usually non dimensional but author included the dimensional options. Once you finish your model and to rescale draw a line of known length, use tape measure tool to measuer it then enter the desired scale. ( Read help center info first) The program will ask if you what to scale, answer yes.
I used you stl model, imported to su, then exported as dae. I then used the MeshLab program ( it has a number of filters one can use clean and repair, re-mesh et., and then inported back to SU . I would not suggest you do that step just because of learning curve. Do not do repeated import / export of the basic stl it can get bloated etc. Use the dae to import back to SU and use that for your edits.
In the future make max use of components they isolate geometry to prevent it form sticking to one another. So like the screws all you need is 1 component, make hub one also. Reading help would be help also.
I assume you will plan on print this. If so there are requirements also. It will need to be a solid model.
When you get to that point suggest you make new post you will probably need more help.