Mine is determined by the quilt. I just did one that was all 6" squares that formed 12" blocks, so I did a 6" border. I'll lay the quilt on the (clean) floor, lay out the border, and then adjust it so I can audition the different sizes. So I'll see how it looks, if it's too big, I'll scoot it a little more under the quilt so I can see a thinner border. Does that make sense?