If I was doing mine all over again I would use aluminum unistrut rails.
With rails you can be sure every screw you use to mount the rail goes into a rafter stud.
Use a stud finder and painters tape to mark the studs. If you use the mounting brackets that come with the panels, you can only be sure one side of the panel has mounting brackets mounted to a rafter stud. The other (trailing) brackets will have to be fastened where they lay.
Also with rails, you can move/replace/upgrade the panels without drilling new holes.
When you are ready to mount the rails, clean the roof with alcohol then put down a 2x4" piece of Eternabond tape where you plan to drill the holes for the rail screws. Next, set your rails and drill the pilot holes for the screws. Put Dicor self leveling sealant in the screw holes and between the rails and the Eternabond tape. Put a spacer (or thick washer) between the rail and the Eternabond tape to allow water to drain away under the rail off the roof. Install the screw through the rail and put some more Dicor over the screw head.
As far as panel size goes, I drew a scale drawing of my roof with all the obstructions on graph paper. Then cut out scale panels of the sizes I was interested in and see how they fit around the obstructions. I went with 200W panels as they seem to fit best to maximize (future, expansion) coverage.
Be aware if you are pre wired for solar the wire is 10 gauge which is a maximum of 30A.
I bought a Victron BMV-712 battery monitor and a Victron 100/30 MPPT. I wish I had gone for the 100/50. The 100/30 max output is 400W (30A).
I have two 200W panels. I find my batteries (552ah lithium) are usually charged by noon or early afternoon.
I also have a DC-DC charger as a backup to the solar.
https://www.montanaowners.com/forums...hlight=charger