What's your method for sizing them?
I use a photo editing and painting program called Krita. It's free, open source and since I'm a digital artist by day it serves me well.
1. Load all pages of the desired stencil .pdf into Krita. The default resolution settings are pretty good for me, since everything under the tracing paper is a little blurred anyway.
2. This will put all of the pages on separate layers, so I then group all these layers by stencil. ex: group 1 will be all the stencils for the right ear, while group 2 is all for the left. You do this by holding Ctrl and clicking on each layer you want to put in a group, then right click on one of the blue highlighted layers, go to Group, then Quick Group.
3. If you right click on the "group layer", go to Add, then select Transform Mask, it will add a transform mask to every layer in the group. Do that for each group.
4. Use the eye symbol next to each layer to hide them until the desired stencil is visible on the canvas. Select the transform mask layer for the group the stencil is in, then in the toolbox select the transform tool. (It kind of looks like a square with a plus sign inside of it).
5. Go up top to Image, then Mirror Image Horizontally. There's a shortcut to this but I don't remember it since it's bound to a button on my graphics tablet. Also make sure you use the "back side" of the tracing paper. Press Tab to go into full screen. Just don't be stupid like I was the first time... (I forgot the full screen button and had to Alt-F4 out of the program.)
6. This part gets pretty tricky if you're not familiar with the program, so I'll try my best to describe it but just let me know if you'd like me to update this post with pictures.
Click on the canvas to begin transforming.
7. You're gonna see a little crosshair in the center of the canvas. Drag that over to a nice corner of the shape you're working on, then tape your tracing paper to the screen, trying your best to keep it flat and rotated the same way as the stencil.
8. Use Ctrl + Middle Mouse Click to smoothly zoom in and use Middle Mouse Click + Drag to pan so that corner we marked earlier lines up with the same corner on the paper.
9. Now use the handles on the edge of the canvas that appeared when we clicked with the transform tool to stretch the image on either side. It's okay if you need to pan away to be able to get to the handles, just don't zoom out or else you'll need to do it all over again.
10. Since we marked that corner, it will always stay in the same spot, since it is the center of origin for the transforming.
11. Simply scaling the image on either side usually isn't enough for me, so you can also click and drag on the edge lines themselves to skew the shape. This is usually the most effective method for me.
12. Once your stencils look nice and lined up, start tracing! Make sure to recognize the fact that the stencils are backwards. I use a white pastel pencil since the mark it leaves is clear and comes off really easily with a slightly wet finger. Mine also has an eraser on it. The only problem is that it does not keep a point at all.
13. Once you've finished tracing, peel it off the screen, take off the tape since it's now on the wrong side, and tape the paper right side up on the helmet.
14. Finally, use something like a ballpoint pen to transfer the pencils on the helmet.
Phew! I didn't notice how complicated my stencil method was until I wrote it all out. The main reason this works so well for me is because I'm so familiar with the program, so I can knock out this 14 step procedure in about 10 to 15 minutes. Please let me know if you'd like me to update this post with pictures, since I really think it will add a lot of clarity.