Ok some may disagree with me.
If it were me I would cut squares of two different colors or prints the same size. Enough to go around your center piece. Make sure you make the same mount of both. Now lay them right sides together, draw a line diagonally down the middle and sew 1/4 inch on either side of that line. Now cut them apart on the middle line and press open. You should have two triangles sewn together into a square. The bias is on the seam line so they should not stretch a lot. use the squares to make your border. It doesn't really matter what size they are as long as they are all the same they should come out a consistant size. I think it is traditional to pick the size block you want finished and add 7/8 of an inch to that measurement when cutting the blocks. The only problem I have with that is that precise a measurement always gets me in trouble. I tend to stick with whole numbers and trim down if necessary.
I also find that adding an inch or even two border between the center and the border of triangles is more interesting, helps the quilt grow and is easier on the eye. A little wider plain border after the triangle border usually looks good .