Those are qualified for wall mounting, so obviously no clearance is required behind them. Part of the consideration is the expected load. They will run warm, but not hot if you stay within the specified limits. I've used a few of this type of dry transformer to get 240 from 480, 3 phase(larger size though). Even loaded pretty hard, they never got what I would call hot. In some of the hospitals I do a bit of work in, the closets hold several floor mounted 480/277/208 transformers. They do get warm, but not hot. The allow about 4 inches behind them, and nothing is near them on the sides or front.
In your case, not knowing the specific load, or environment, I would say you would be safe doing as TD says. 8 inches all around, except the back, if you do wall mount. You could get by with less if necessary, but it would be good to have some air space between them.