What would be a general rule of thumb for the size of a steb center,
compared to the diameter of the spindle you're turning?

Let's say, for instance, that I'm turning a round 1 1/2" diameter tenon on
the end of a 2" x 2" square piece. I see centers that would fit my lathe
listed on the web with widths of 1/2", 7/8", and 1 1/4". Which would I use,
and why?

Thanks,
Tom