I have an 07 Devinci Sydney. I want to put some 45mm wide tires on it. Does anyone know if this is possible. I went on their website and the best I can do is find the max width of the tires for the frame of the 06 model (which is 35mm). Does anyone know what the max tire size means? And where to they get this number, by my rough measuring, my frame should be wide enough for at least a 50mm wide tire. There must be something that I don't understand.

There's a distinct possibility that the tires would rub on your brakes too. I'd say the manufacturer's spec is accurate within a few mm.