Really how big you can go is going to be determined primarily by the brightness of the projector you are using, plus your seating distance and preferences.
At Panamorph we use screen height to determine throw ratio, rather than width. Using height eliminates the confusion around whether or not you use the 2.35:1, 2.40:1 or 16:9 width. Typically we recommend a throw ratio of at least 2.85X the screen height, which should also work if you end up with the Prismasonic. It is a bit safer to judge from the front of the anamorphic lens, since longer throw distances are almost always better for anamorphic setups.