what size potting medium for Brsdm.?

[boxerfan]

My Brsdm. has roots growing out of the bottom of the pot, s I'm taking that as a hint that it would like to be repotted when it finishes flowering. The current medium looks like peat or maybe even potting soil. I have a Brs. and 2 paphs. that are potted in fine medium (1/4"-1/2") and seem to be doing well. Is this similar to a seedling mix? I'm most likely going to be ordering over the internet, so I want to be sure to order the right medium the first time out. Thanks, Lisa