If all you have is an STL file, that may be a bit of a problem.
First of all, Solidworks has a limit as to the size of the STL, namely the number of triangles, is about 20K. depending n the amount of detail of you STL, that may exceed Solidwork's ability to import the STL as a solid.
You would have 2 options here.
1. import the STL into something like MeshLab, then reduce the number of faces by running a simplification filter.
2. Open the model in SolidWorks as a Surface body instead of a solid.
In either case, making modifications to the model may prove to be very hard if not impossible. If all you want to do is scale the object, that's simple enough, but if you want to modify it's geometry...well, good luck
Anyway to scale it, got to Insert/Features/Scale. The select Centroid and the scale. If it's to be scaled uniformly, then enter a value. For example to make the model twice as large, enter 2. If you want to stretch the model non-uniformly, then uncheck that box, and you'll be prompted to select different values for X, Y and Z.
To print or, 3D print, that is the question...
SD3 printer w/too many mods, Printrbot Simple Maker Ed., FormLabs Form 1+
AnyCubic Photon, Shining 3D EinScan-S & Atlas 3D scanners...
...and too much time on my hands.