The Base & Cap System comes in 8, 16 and 24 ft lengths. To achieve a good yield from the 24 ft. lengths one might use 16 ft. lengths on the roof and 8 ft. lengths on the side walls, of a freestanding greenhouse. If you have 10 ft. roof sheets and 6 ft. side wall sheets use the cut offs on the end walls. It is fine to have joints between smaller lengths of Base & Cap on vertical walls.
When selecting the best length for your job try to avoid joints on the roof.
The best way to cut aluminum is a power mitre box with a multi tooth carbide blade, specifically for aluminum. Home Depot has some inexpensive options.