The problem is that since he's a mixed breed, he could have a variety of traits from each breed. There's no guarantee of anything.

As far as the hair cutting, both shih tzus and poodles require regular grooming. Is his hair more like a poodle or a shih tzu? If you plan on keeping him long you must brush him TONS...both breeds get matted easily. If you don't have time for that, I'd take him and get a trim regularly. I'd suggest visiting a groomer even if you don't get a trim, they do other things such as clean the ears/clip the nails.

For the size can just figure out the approximate weight from the weight of both a mini poodle and a shih tzu. Give or take a few pounds from the average weight of each breed. Does he have fairly long legs like a poodle or the shorter legs of a shih tzu? That could tell you the approximate height.