Width: The big number. Narrow free-standing and countertop models vary substantially. Measure the opening, then compare it with the current exterior dimensions in the manufacturer listing.
Ventilation: Built-in installation generally requires a model designed for front ventilation. Free-standing clearance varies, so follow the current installation manual rather than assuming it can sit flush with a wall or cabinet.
Bottle count vs. drinking pace: Buy enough capacity for the bottles you keep at home, plus room for the next purchase. Published counts usually assume standard Bordeaux-shaped bottles; wider bottles can reduce usable capacity.