Whether you go Ku or C Band, more surface area = more signal.
36" or better for Ku will make you happier in the long run.
C Band rocks, but you need the space. 6' is a kinda recognized minimun. A lot of guys are experimenting with smaller dishes for C Band, not really beginner projects.
As far as a motorized Ku setup goes, the world is full of compromises. It works great, but does take some time going from east to west. To some, ( but not me!) a motorized dish has a lot more eye appeal than 10 or 12 dishes of all sizes.
Most folks suggest a fixed dish aimed at 123 or 97 to start, then motorize it as you become comfortable with the technology and want to delve deeper into the hobby.