Flower Arranging is an art all its own and depends on a lot of things beyond just the vase, but I'll try to offer what help I can. For roses, especially that many of them, you're going to want something that still exposes around half of the stems or maybe a little more, which also gives them more breathing room and lets them fan out a bit. You also want to avoid a square vase, otherwise the stems will tend to clump in the corners, which isn't ideal. An Adrift Vase might work well with a dozen roses, since they come in a variety of colors and have a rather open 'mouth'.