Built-in wall ovens can look intimidating because they're, well -- built in. The truth is, though, they are not difficult to replace once you undertand how. Wall ovens come in four standard widths: 24", 27", 30" and 36" and slip inside a cabinet. Unless you intend to replace the cabinetry as well, you'll want to choose a new oven that's approximately the same width as the one you're replacing. Small alterations to the cabinet opening size can be made without a lot of difficult and, in fact, this video deals with that situation.