I recommend looking into the American DJ brand. When I researched years ago ADJ kept coming up as a high quality brand, I ended up getting one of their machines and its now about 10yrs later and it still works like the day I got it.
Size wise, I recommend getting a decent size, you can always turn it down or use less dense fog liquid, but if you ever need to use it in a place that has a draft or one day want to use it outdoors you will appreciate the additional capacity. I have used mine for studio work, music videos, and other projects.