Yip,
As for the head diameter, it depends on the width of the rear extension on the guard. One I am working on now has a rear extension that is 1/2" wide and I'm using a #8 screw. #8 has a 5/16" head diameter. As you probably know, screw size just refers to head and shank. Length is a separate measurement.
A #8 on a 1/2" wide guard gives enough margin on either side to look okay and still have some strength, so that the guard isn't so likely to break at that point. That's my opinion.
As for length, I try to find a screw that will have at least 1/2" inch of bite, and I think anything longer than 1" is overkill. So I think 8 x 3/4 would be about right. Again, just my opinion. This place is a garden full of opinions, so others may think differently.
I hope this answers your questions.
I'm glad to see that you didn't really give up and just go to working jigsaw puzzles (as you suggested you might in your post about making straight lines).
