there is an interesting article about height evolution in the NBA:
70 Years of Height Evolution in the NBA [4,504 players analyzed] | RunRepeat
the key findings are:
Point guards are the tallest they have ever been in legue history, while the average NBA player has never been shorter over the past 39 years, all positions outside PGs are shortest sice the 80s.
similar to height evolution, point guards are the heaviest they've ever been. all other positions are getting lighter.
there are some interesting diagrams, the one that I was especially interested in is: "share of players in the NBA per height range", because this shows the trend best: a significant increase of the share of players from 6'3" to 6'5" over the last 20 years. this has to be the increasing number of combo guards teams play in their back court.
I think noone claims that a small player can't succeed in the NBA, if he has some outstanding skills. but position less play and switchablity generates a trend and I don't think teams totally ignore that trend. the point is, you want a player, who can defend at least two positions, better three, if not four. if you can only defend one position (the tiny guard, the plodding center) you inevitable block many possible defensive shemes.
that's why I don't agree to the theory, that because you have Wemby, it's not a problem to play a small guard, because Wemby will cover for this. yes, right, Wemby would minimize the damage, but you still rob yourself of some defensive versatility. it's like saying "hey, we have a hole in our boat, but no problem, we've got a guy with a big bucket"