Hi Antoinette, here are a few more great tutorials that will show you how to make a frame - https://www.apartmenttherapy.com/large-picture-frames-you-can-make-on-the-cheap-242167
I can't remember the exact size of this frame, but my husband built it to surround the mirror It is pretty much square and at least 40' .. He just determined the size and used wood that he cut the corners of so they fit together. Measure your art work and use the outer measurement of that to fit the inner size of your frame. Size of outer size depends on width of wood you choos.
Hello Antoinette, I hope this video helps you
I made a DIY frame for my home’s bathroom mirror, which is pretty large. I used trim from Home Depot. You can see my tutorial here: https://momhomeguide.com/diy-bathroom-mirror-one-room-challenge-week-4/