Marshall Dugger III wrote...

I intend to make it 24" deep but was wondering if there is a "rule
of thumb" for the width?

Whatever will fit in the storage space you've set aside for it.

It is handy, on occasion, to have at least one side overhang the edge of
the saw table by an inch or two, for clamp space. It also makes squaring
the fence to the sled very easy: you can screw the other end down and
clamp the free, overhanging end for test cuts. The screwed end provides a
pivot. Once the fence is square, simply flip the sled over, drive the
remaining screws, and remove the clamp.

Cheers!

Jim