How to alter an STL part design, or easly read dimensions for redesign?

I use Autodesk Inventor 2015 Pro, and a Printrbot 1405 simple wood. I'd like to print out a new base that will enclose a Full Graphic LCD and a fan for the Printrboard.

Is there any way I can get the original model files, or convert what's available online, so that I can design my own part using the dimensions from the originals (screw alignment etc.)...?

I once found a 3d file pack for the Printrbot 1405, but I deleted it because it wouldn't open with dimensions in Inventor.

I'd appreciate any comments viewers may have that will help me.

Thanks

Printrbot files ar available at
[printrbot.com]

I myself use autodesk 2014 when I make designs and with the Mesh Enabler plugin I am able to import and edit .stl files pretty well, but i'm not sure if it works with inventor 2015 yet, and sometimes it can have some trouble on certain models

DesignSpark Mechanical seems to be able to do measurements from and edit STL meshes. I just discovered that a couple days ago.