what should the opening be for a 30 X 26" recessed glacier bay medicine cabinet model # 4450

Check "specifications" of specific medicine cabinet you are interested in. Recessed opening sizes given there.

That is definitely an odd size medicine cabinet. Usually, medicine cabinets are 14" wide - to fit between two studs. If it is 26" wide than you will have to do some framing to fit it in.

The opening should be around 26 1/2" , side to side

Milo

Here are the specs on your medicine cabinet: http://www.homedepot.com/p/Glacier-B...specifications