I have also heard good and bad on Visions, but you're going to find good and bad reviews on everything. Personally, my favorite acrylic cages are Animal Plastics. Have been working great for my animals. But anyways, Visions are perfectly fine.

To answer your question, those dimensions are perfect. Actually, that's just a few inches smaller than what I keep my BPs in.

Standard heat mats aren't going to heat through an acrylic cage. Flexwatt is definitely the way to go.