The diameter of the fuselage is quite small, so it did feel small when you got on board. A little tighter than a 737 I'd say. The overhead lockers, as in most short haul aircraft (yes, Concorde was counted as short haul, given the longest trip was about 3 1/2 hours!) would prevent people from fully standing up in the window seat positions. The aisle was fine though. The seats were nice though , 2 - 2 configuration, I think 34" or 36" pitch (?), plenty of room for your knees and wide enough for comfort and not jostling your neighbour. Good seat cushions too, so I found it quite comfortable for the length of flight.
Often on a lightly loaded plane, the seat next to you would be kept empty.
A delightful experience, very glamorous, I'll never forget it