yup, you dont need a 12" to see the grs thats for sure. I have a 8" and if the grs is on the correct side of the planet then i can see it regarless of seeing usually. just be sure that if the seeing is bad or jupiter is low towards the horizon you back teh magnification off a bit
i can spot it at 80x magnification which is a 15mm eyepiece in a 1200mm focal length scope.