I have been using Blick floater frames. I looked briefly and the smallest size I saw was 8" x 8". Here’s a link: https://www.dickblick.com/products/blick-wood-floater-frames/
Here’s a link to pictureframes.com where you can get custom size frames, including floater frames:
https://www.pictureframes.com/Custom-Frames/Natural-Wood-Custom-Floater-Frames/CF5-black-wood-canvas-floater-frame
Hope this helps.
Judy