How do you figure out how much to scale something?

I've downloaded several files over the past month that have obviously been smaller than "normal" size. Most of them have instructions on how much to scale them, but what about STL files that don't list the scale size, how do you figure it out?

One file I found needs to be EXACTLY a certain size and it's way to small, so how do you figure it out?

A lot of the stl viewer programs will give you the X, Y, and Z size of the STL. You could do it from that. On a Mac, Pleasant 3d will give you the dimensions and do the scaling.

netabb basic has scaling built in and a measuring tool. You can even specify how big one of height/width/depth an object should be and it will scale the rest in proportion.

I've noticed that Slic3r won't let you do decimals. I had something that I had to scale 24.3 and it wouldn't let me do the .3 or am I missing something? I just used Netfabb basic for the scaling since the person who made the file was nice enough to include the scale dimensions.

As for making my item the correct size, I've got Slic3r, Netfabb, Sketchup and a few others. Let't say I download a file (AR lower receiver) and it's 19.6240mm in Sketchup, how do I go about making it the correct size of 7.503 +- .006 (190.58mm) since obviously gun parts need to be accurate?

If you multiply 19.624x9.712 you get 190.588288, so does this mean that I need to scale it by 9.712?

I know Uncle Bob and I have had this conversation before about 3d printing gun parts and having them work properly, and you said it's probably not going to work. I understand that, but I still want to try anyway. I printed out a magazine the other day and it looks awesome. I'm going to dip it in acetone vapor and see if I can even load it. I would like to try the same thing with this lower receiver, but am struggling with getting it to the correct size.

I wasn't near my programs this morning, so I couldn't try it, but I just opened Netfabb basic and it seems to work so I guess I'm on the right track.

You want to use a CAD program rather than a slicing program. They look after the scaling math better on most of the CAD stuff.