Landon, I generally go with 3/8" for the spacer. It seems to fit most blades larger than five inches for me. Plus that is a standard size, 3/8" x 3/4" x 12" for my 416 barstock. However when it comes to more complicated spacers like three piece I have to adjust things a bit. You can always use the golden mean to give you a starting point.

Brion

Thank you Brion, I was wondering if you could use the golden mean rule on spacers, or if there was a variant of it for spacers.

Landon,

I would agree with Brion if the handle has a thin (or no) butt cap. However should the knife handle have a lengthy pommel then a longer spacer seems to balance better.

I do not have any rule of thumb for this, although I seem to gravitate toward making the spacer the same width as the ricasso. Because I always work from a full scale design drawing, the dimensions always vary and I always have measurements that "look right." Visual balance is the key, whether you are balancing against a buttcap as Gary mentioned or balancing against the guard or the ricasso. Brion's suggestion is also valid because you can use the gold ratio in almost anything. The human brain is somehow wired to find this ratio and focus on it. The simple fact that we find it pleasing is a bonus. It would suggest making the drawing, getting it to look right and build to the drawing.