Will I cause any harm by driving with 2 different sized tires???

I have a 1988 Chevy Astro Van. It originally had 215/75/15's on it. Recently I got a great deal on some 205/65/15's. I put the two on the front right away, but before I could find a matching one for the rear, one of them blew. So now I have one of each size(listed above) on the rear.