This is probably a Fusion 360 problem but when I model something in Fusion 360 and measure it all is good and the correct size.
When I export it as an STL then import it to Simplify3d it is MASSIVE like 20 times to big.
This is the opposite behavior of what you would expect if you were modeling in inches instead of mm.
Not sure about the Windows version, but the Mac version of S3D assumes your model is in mm, and will ask to scale it up if it's too small. There's no way to change that to go the other direction; if a model is too big it will just warn you that it doesn't fit on the bed.
STL files themselves have no units - the scale is arbitrary, it's just a bunch of numbers - so a model could be defined as "24 units wide", but it's up to the slicing software to determine if that's 24 inches or 24mm. If S3D comes across a model that is really small, like 4 units wide, it assumes it's supposed to be inches and asks if you want to scale it up by 25.4.
That all being said, if you go into Preferences for Fusion 360, you can set it to use inches or mm under the "Default Units" section. When you freshly install it, it's set to mm. What are the dimensions of the object you're modeling? Are you sure it's not just huge in Fusion 360 and you're zoomed out?