For my 6x12, I did two things:
1) I added pulleys attached to the ceiling joists, 1 foot beyond each corner. The cable simply threads over the existing pulley wheels in reaching the new pulleys. That got me the 6' width.
2) To make the 12' length, I extended the axle connecting the two end bridges. That took some "engineering" - I purchased a steel square (hollow) 1"(?) rod to extend the length, from Home Depot.
Its been "up" for about a dozen years now. I park my Mustang beneath.
Ken