Go larger. A 16" usually only gives you about 12" of quilting space so you may be hard pressed to do a single average size block in one pass and you definitely won't be able to it if the 12" finished block is set on point. Like Lyric Girl I went with 24" right off the bat. Now I rarely use all that space but it sure is nice having it for when I do. I think a 21" throat would be ideal. Also get the biggest size rack you can fit. I could only fit a 10' rack in my space and there have been plenty of times I wished I had a longer rack.
Speaking of racks, give as much thought and consideration to your rack as you do the machine itself. A good solid rack, preferably with a deadbar can make all the difference in the world. My machine is an Innova and the rack is unlike any other LA rack on the market. It is infinitely customizable to your needs. It has a deadbar so no need to raise the takeup roller as you advance the quilt and it has a feature no other LA rack has, the ability to lift the top roller after the quilt is loaded to ensure you batting stays nice and smooth. And it is made out of aircraft grade aluminum so solid, solid, solid.