It seems like Vatican museum exit provides a short path to avoid the long lines of visiting St. Peter's. But if I rent an audio guide, or check bags at the museum I will have to return to the entrance of the museum, is that correct?
If so what is the size limitation on checking bags? How can I avoid checking bags? Is there an audio guide return at the exit?
If you check a bag, a long umbrella (yes - they will make you check those also), "borrow" a wheelchair, or get an audio-guide, you must return these or head back towards the Museum entrance area... so No, you will not be able to use the "group" exit and head over to St. Peter's Basilica if any of these situations apply. Many tour guides will "gather" their headphones while in the Sistine Chapel which allows folks to decide if they want to cut through the Basilica or head back into the Museum.
Usually a smaller backpack will be okay. But if it looks "BIG" you will be redirected by the folks at the X-Ray machines to head to the "coat check" area. If I was carrying a backpack I'd enter the doors and head to the busiest X-RAY as far left as I could go - that's the furthest AWAY from the cloakroom. There you might get by, or could "shuffle" into the crowds as you leave the X-Ray unit... I've seen kids do this a few times.
A good picture of the staircase leading down from the Sistine Chapel to the Basilica is located at the bottom of this post: Vatican Line Info .
I would suggest going to St. Peter's in the morning and checking your luggage there(the lines are very minimal or they were for us in August at around 7:30 am) then head over to the vatican do the tour, take the St. Peter's exit visit the church and then pick up bags.
