This is one of those things that i've thought about quite a bit, but never really got to figure out....I think the longer bar is a no-brainer; i went back and forth with the idea of using a slightly larger-diameter caster wheel, but I think that's got more to do with my gut instinct than it does with any actual design concerns.
I wish I had more to offer; tbh, i mostly responded to keep this thread up top and lively, hoping to encourage more discussion....