Mine is 10 years old, so the cost installed wont be much of a help to you, but I went 6x6 with a 6x6 carport on the front of it (I new the missus would never get out to open the garage door), with an extra metre of height. Portal framed as I new I would never want to line it, as well as making sure there was a full 6m long span in the car port area so there was no upright at 3m for kids to bang car doors against.
Would have liked it longer, (as my boat and trailer wont fit in the shed, so it lives in the carport with the draw bar sticking out) but as I went portal frame, the shed could only grow in 3m lumps, pockets were not that deep.
Still pretty happy with it.