I understand this reply is late, (March 2021) but I was on the replica Grand Staircase in the Titanic exhibit in Las Vegas in 2019 and it felt quite roomy. I've no doubt it was perfectly adequate for the ship, aside from when it was in use as an escape route,Take captain Smith all the way to Officer Moody, 7 men roughly the same height and weight as each other. You can find the stats under- (Murdoch) (find it on here somewhere) those men are roughly the same build, height, weight. Very slim, very light - average weight is around 58 kgs - 68 kgs and their average height was around 5 foot 7 to 5 foot 9 - (only 2 were 6 foot) although slim build . Today's Standards that's small. Most men today weigh over 100kgs and stand around 6 foot plus.
Even Looking at the grand-stair case gives me cloasterphobic.