How do I keep the .stl files from being resized while exporting?
.stl Basics
The .stl format has no inherent sense of which units you use. items are to scale to an ambiguous 1, which could be 1 meter, one millimeter, one lightyear or one inch. To a .stl, only the relative sizing matters. All these faces you see are compared to a line with the length of 1-unit that is
Slicer-Modeling Software interaction
The most common graphic design programs export in millimeters, but some US ones just assume inches, which is a factor of 1&quot;=25.4mm.
Cura, Netfabb, and Slic3r expect that the 1-unit line is one millimeter long - but if it is an inch instead, then the model is shrunk by 1/25.4 or to about 4% of the right size. Scaling up by 2540% one would return to the millimeter scale.
But then there are other programs that use other choices of scales. Blender for example assumes a scene is in meters by default.
Inventor
Inventor can export .stl in a variety of scales, which all just serve as how the length of the inherent but invisible 1-unit line is drawn. The default choice is centimeters, so a scaling factor of 1cm=10mm, which would explain the models being only 1/10th of the expected size in Slic3r. to change the scaling, follow the manual:

upon exporting a .stl, click Options
under Units, choose mm