Velit Rooftop AC Units are designed to fit through a 14 inch by 14 inch hole (or 35.5 cm by 35.5cm), but some trimming may be necessary depending on what is used to cut the hole (Saw vs angle grinder, how thick of a line you trace/cut, etc.) It's important that the hole is fairly square, as the bracket and support bars are going to squeeze down on your roof material to secure the unit (everything outside of the thick black lines here would be roofing for example): 
You can see how you need enough roof to "clamp" onto! 3000r and V3 systems have even less room for error. Their mounting bracket is a complete square and has a small lip for clamping onto the roof: 
You can switch from any brand which has an installation hole smaller than 14 inch (35.5cm) in either dimension. For example, you can switch from a unit with a 10 inch by 12 inch opening.
As far as interior clearance. The models 2000R and 2000RMini use an air distribution box which is 11 inches by 14 inches (or 28cm by 35.5cm) And the models 3000R and V3 use an air distribution box which is wider, at 19.5 inches by 20.5 inches (or 49.5cm by 52cm)