>>7297256The size vary for different 'grades' and for different models.
Excuse the garbage photo, but here's a little comparison. The Mokou is 1/8, the mini RX-78 is a "Real Grade" but I don't think there are many of those. The crossbone beside is a master grade. 1/100 scale maybe?
They are easy to put together. You can mess up I guess, but it's still kind of forgiving. The instructions are in Japanese but they are all pictures essentially and the parts are numbered, so you don't need to know any to put them together.
Manufacturers, I guess for Gundam specifically you'd just get Bandai. Kotoubukiya makes some kits as well but I don't think they make Gundam ones. I started a new armored core one yesterday.