I miss a smaller Spot AF autofocus frame size in every Canon mirrorless full-frame camera except for the model EOS R6, because this camera model currently has the smallest Spot AF autofocus frame size.
How do I know that?
Some time ago I sold my little used EOS 5DS R, and I no longer have the option to make a direct comparison with my EOS R. Fortunately, there are RAW files left on the disk, and the ability to download RAW files from the EOS R5/R6/RP from the Internet.
I did a comparison of the dimensions of the autofocus frames.
Software: Digital Photo Professional 4 (DPP)
Conditions: Enlargement of the photo in DPP to 100%
Measurement type: Dimensions of the autofocus frames measured at 100% magnification of the photo. I used a normal ruler touched to the monitor screen. Height x width in millimeters. Estimated measurement error at the level of 0,5 mm.
Spot AF EOS R6
61 x 50 mm
Spot AF EOS 5DS R (with this small square in the center providing better precision for the selected AF point, this small square is not shown under the DPP)
65 x 65 mm
One-Shot AF EOS 5DS R
65 x 65 mm
Spot AF EOS R5
67 x 67 mm
1-point AF EOS R (smaller AF point frame size; after pressing the cmd + i key combination it is detected as „1-point AF” in „AF area select mode", not us „Spot AF”)
65 x 72 mm
1-point AF EOS R6
100 x 93 mm
1-point AF EOS R
102 x 109 mm
1-point AF EOS RP
111 x 111 mm
1-point AF EOS R5
111 x 134 mm
FlexiZone - Single (Live View AF) EOS 5DS R
223 x 167 mm
Autofocus frame size in AF mode „Servo AF” with AF area select mode „Face detection + Tracking AF” is smooth, and probably depends on the distance to the target. I have some RAW files with one model taken at different distances on my drive (from some R5 test from Jared Polin), and each has a different frame size - sitting on the ground full body 65 x 65 mm, to the hips 74 x 74 mm, bust 86 x 86 mm and head & shoulders 113 x 113 mm.