It isn't as significant as it used to be for sure.
Someone above said we should by aiming for a stadium the size of Uniteds which is around 75k, but if we get to 60k... Is 15k less such a detriment in the grand scheme of things? I don't think so, especially when you factor in our commercial deals and general merchandise sales compared to clubs in and around us like Chelsea, City, Arsenal & Spurs.
As said already, there's only a handful of clubs with a bigger fan base than ours in the world which more than makes up for 15 thousand seats