This small guide shows how to scale a model in Source Filmmaker.
No extra tools are required as the source engine already supports this feature out of the box.
It does use the 'Element Viewer', so be careful or you can crash SFM and break your project.

Add your model to the Animation Set Editor.
Rightclick on the model, click on "Show in Element Viewer" → "Model".

In the Element Viewer you should see the tag "transform". Open it by clicking on the (+) on the left of the name "transform".
Select the tag "transform", rightclick on it, click on "Add Attribute" → "float".


A small window should pop up. You can select the type of the attribute again (it's a float) and change its name. The default name is "newAttribute", change it to "scale". Click OK to add it.

As you pressed "OK" from the last step, you will notice the model has disappeared. Don't worry, thats because the default value is 0. You can change it by clicking the number next to "scale" in the Element Viewer and then you can change it. 2 is the double size, 1 the orginal size and 0.5 is half the size.
Bones will also be resized, so animations and sequences should be working fine.

Left Image: Scales from left to right: 2, 1, 0.6, 0.5
Right Image: RED Sentry and RED Engineer are at scale 1.