For picture frames, I highly recommend the miter sled instead of the miter saw. It is much more accurate and easier to use.
As long as you cut the ends on the opposite sides as shown in the video and use stop blocks for the length of the sides, you will get perfect frames every time.
It doesn't have to be as fancy as the one in the video, in fact, mine does not have the front and back fence at all!
I just used a piece of plywood, two runners, and two 45 degree fences. Just make sure you never cut all the way through the plywood base.
As long as the fences form a perfect 90 degree corner at the front and are straight, you are good to go!
Much cheaper, easier to use and a guaranteed true miter corner every time, regardless of the size of the frame!
Rog