the border should be in a proportion of the block size of the quilt- so if your quilt is a sampler with 12" blocks and you want to add borders they should be able to be evenly divided into 12-to be proportionatly balanced...you have lots of choices and can use multiple borders, to get larger sizes and keep things balanced. for 12" blocks you can use 6", 4", 3" or 2" with a good balanced look; 15" blocks - 7 1/2", 5" or 3"...