Al really isn't small for the C position in the NBA. There is a myth that most C's are 7'0 and it just isn't true. I believe the average height for NBA C's is right around 6'10, noting that that number is skewed by the number of 7'0 guys who are in the league mostly because they are 7'0, but because they have exceptional basketball talents.
Not quite.
If the average height is 6'10, then there should be quite a few centers 6-9 or shorter. who are they? Lets look at only the starting centers if the talent issue bothers you.
6'10 is at the shorter end of the height range for centers(even if you take only the starters). If you take atleast the premier teams,
Shaq- 7'1 Perkins -6'11
Bynum - 7'2, Gasol-7'
Yao -7'6
Dwight -6'11
Oden - 7'2
Haywood -7'0
Bosh - 6'11
Hilario - 6'11
Noah - 7'0
Bogut - 7'0
No one on the above list is 6'10. And they are'nt pogo sticks either. They possess a good physique as well, atleast most of them.
There maybe one or two good teams at best(like Thunder) that have centers who are 6'10 or shorter
Both Millsap and Al are definitely undersized at their positions, maybe Millsap more so.