Today's modern movie theater seats are on average 22-inches wide, measured from the center of one of the chairs' armrest over to the center of it's other armrest. Back in the 1990's AMC used 20 inch wide chairs throughout the USA, but in the last 10 years they have converted their theaters to the wider 22 inch because, quite frankly, American have gotten wider. Muvico also uses the 22 inch as its standard width. Now that is not to say that there are some exceptions. VIP seating areas in theaters, especially theaters offering stadium-style seating, is become more and more popular. (They usually cost you a couple of bucks more when you buy your ticket too.) These chairs are usually 24 or more inches wide. Check out the dimensions of typical theater seats at http://www.celebrityseating.com/product.php?page_id=Cinema-Star-Rocker-Theater-Seating where you can see a 22, 23, and 24 inch wide theater seat.I think your step-dad will do fine in the 22 inch wide chairs. Most home theater owners who put real movie theater seats in their homes choose the 22 inch model. Plus theater seats are rated to hold up to 450 pounds.
Multi-plex movie theaters have auditoriums ranging from 85 theater seats to 300+ theater seats. New movie releases are first shown in the larger auditoriums and then after a couple of weeks are moved in to and shown in the smaller auditoriums, thus making room for the next 'new releases'. Unless it's a movie like Avatar, which stayed in the large auditoriums for 2 months!
Hope that help answer your question.
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