While Dunkirk's running time is yet to be confirmed, the official screenplay clocks in at 112 pages which, going by the popular held belief that one page equates to a minute of screen-time, suggests that Dunkirk could be Nolan's shortest film since his debut Following which clocked in at 70 minutes (his next shortest, at 118 minutes, is 2002 psychological thriller Insomnia).
What's more likely is that Smith's statement that the film will feature a small amount of dialogue means that the script is shorter, indicating that Dunkirk could well have a run time more similar to his longer films such as teh Dark Knight trilogy, Inception and Interstellar which all run between two to three hours.