Well, it isnt that you cant do a king size, its just that you will be limited to quilting it 4.5 inches at a time..so this limits what you can do..there are many 4.5 inch pantographs.. and free motion can be done in 4.5 inch increments.. but not so easy to make them look continuous..
I have just aquired the Grace Pro and I like the frame. It says that it will take up to 16 inch machine,, but Im wondering about that too. I would like to upgrade to either 16 or 18 inch machine.. so I am right there with you as you seek out this info
With 16 inches, you should be able to do free motion on a 12 inch block with no problems..
I dont know anything about the other frames on the market,, hopefully some others will..