Well, this turned out much more difficult than I hoped and I had to experiment for some hours. I hope you understand the following, if not ask as precisely as possible!
Part 1: extracting the model (can also be done using the model editor, but I prefer war3viewer somehow)
-open war3viewer
-open the mpq the model is located in (in case of the pandaren war3x.mpq in the war3 folder)
-find the model of your choice in the structure window (make sure it's in MDX-Mode). If you don't find it and need the path, go to object editor and look at the custom path part of the model file. Then you still have to guess the mpq, though.
-right click the model in the structure tree and choose "extract MDX". Save it where you want it to save.
-close war3viewer
Part 2: Scaling of the model
-open War3 Model Editor and open your extracted file (over the file menu), in our example PandarenBrewmaster.mdx
-go to menu Windows and open the model editor
-go to menu Selection und choose "Select all"
-go to menu Transformations and choose "Scaling"
-enter the scaling value you want to use in the different axises. For proportional scaling, use three times the same value of course. In our example, we want to scale down the pandaren, to let's say scaling 50 percent of its original size so we enter 0.5 as X-, Y- and Z-Scaling.
-Make sure the pivot point is 0,0,0 and the centre of scaling set to "use pivot point"
-if you save, import and try out the model now, you'll maybe see that the animations behave kind of weird, if not, you're lucky and can skip the next part
Part 3: Additional edits
-find an animation in which the model is invisible at the end using the animation controller (menu windows), in our pandaren example "Spell throw" does that job
-open the sequence manager using menu Windows
-double click your animation, in our case spell throw and copy the value at "To:"; click OK
-remove all animations and create a new one called "stand" with right click; leave all options as they are
-create a new animation called "death" but in this case insert the copied value in "To:". Also enter 10000 as "Move Speed" and activate "non looping"
-if there are parts of the model visible you don't want to be visible (e.g. team glow), select them in the model editor (team glow are mostly only big planes) and scale them to 0
-to remove these speed lines you only see ingame when the model is moving, open the Node Manager and there remove all "ribbon emitters" with rightclick and remove (they have a red symbol and are always included in a bone)
-close the model editor and in the main window go to the file menu "Save as" to save the edited (scaled) model
Now you can import it in the WE and test as missile. To have an explosion or something like that at the end, I recommend just to trigger it using a special effect.