The 66 also has the horizontal bobbin.
I was estimating the size of the pattern. I know on the Juki 9" (it's actually about 8.5") it shrinks very fast. You could probably calculate it something like this:
smallest harp dimension - (diameter of bar +( (width of sandwich x 2(number of layers per roll)) x number of rolls to traverse the length of the quilt to the last advance)) = largest space in which to put a design.
That's not exactly right - the number of rolls per advance will change somewhat because the bar will be getting fuller...but it would give you an idea...
If you plan your designs in such a way that you can split it top to bottom, you can make a larger design.
For instance if you draw the spine of a feather close to the bottom of the space you can quilt in, you can quilt the top of the feather in one pass, then the bottom of the feather in the second pass. I managed to quilt a 14" feather this way when testing. Where it gets to be less fun is in on side border designs for instance where you advance once per feather frond.
I have seen people turn out beautiful work on the smaller throated machines. I'm not that good. I opted for a larger throat.