The best resource it to just check out www.totousa.com and review the spec sheets for the toilets you are interested in.
Keep in mind, the building code requires a MINIMUM of 15" to BOTH sides of the centerline of a toilet, so the actual width of the toilet isn't that big a deal, as pretty much ANY toilet should fit within that centerline minimum width of 30" (or more, if it isn't centered).
As to height of the tank, one-piece toilets generally have a lower tank top height than 2-piece toilets (but cost more). While a toilet doesn't require all that much maintenance, it would be a major pain to have to remove it from the flange to say change the flapper valve. You may want to look into ways to either hinge or remove the shelf for that eventuality.