There are two main places to rent skis at Big Sky. Both are up by the ski area. One is ran by Big Sky and located in the same building as the ticket office. They offer standard rentals as well as upscale rentals including fat skis.
The other place is Lone Mountain Sport, (406) 995-2313, in the Arrowhead building by the Summit Hotel. They have a great variety of skis for both renting and demoing. This seems to be the most popular place for good skiers to rent skis.
A new company, Black Tie Ski Rental, is renting skis there this year. I have not idea how good they are. Their website is http://www.blacktieskis.com
An off-beat place to rent is Mad Wolf, (406) 995-4369. They are located down on Hwy 191, 10 miles from the resort. I've heard good and bad (mainly good) about them.
As for girth, what you saw in 2005 on the tram is now the everyday width at Big Sky. Tram skis are even wider. Something in the 80s is the norm for a Big Sky all-mountain ski with something in the 90s being for off-piste & powder and something in the 100s for macho tram powder skis.