What is the easiest way to modify/resize an object inside a STL file?
Many resources are available for modification using 3D CAD (including learning a tool such as sketchup, which is VERY beginner friendly). The easiest and fastest solution to your particular problem may be the reddit community "3D Print My Thing" which was created for EXACTLY this type of situation (help with modelling parts which will be 3D printed.) Another potential useful reddit community is "3D Modelling" which will have many people who are able to help you with this quadcopter/Go-Pro attachment 3D model.

Third solution: Thingiverse has a customize option for 3D models (including this one). have you explored using this interface to edit the model? This is most likely the easiest DIY solution.