Not necessarily
If you say "in general, this attribute is true about this particular race" then that is not prejudice, because: a) it doesn't treat people badly; b) it may just be a true statement of fact; and c) the fact that it's a generalisation is explicitly stated.

As an aside - even if the speaker did not state that it was a generalisation, do you know any black men who would be insulted that someone presumed they were well-endowed? :) It may be a generalisation, and certainly not true in all cases - but your specific example is largely thought of as a positive thing.
(EuroSong I love horse meat: best of all the animals, Thu 1 Apr 2010, 20:49, 2 replies)

actually
the connotations of it in that context could be construed as negative since the time and place that generalisation sprung from meant it negatively- large genitals were linked in popular imagination to uncontrollable sexuality (raping white women, animal-like, savagery etc.)

It's different now obviously, but the point stands that generalisations even if superficially positive are not necessarily so.

//takes history hat off//
( Amberl was stripey and dominated Europe, Fri 2 Apr 2010, 0:34, closed)

I was not aware of that connection
Weird for people to associate large genitals with sexual assault.

It's like saying that if someone's good at maths, it means they're likely to be a tax fraudster! Or if someone's a fast runner, it means they put that skill to use as a mugger.
(EuroSong I love horse meat: best of all the animals, Fri 2 Apr 2010, 20:09, closed)

Well it's the past
people weren't rational, but certainly that particular belief that black men were wildly oversexed resulting in sexual violence was around until the 1950s. It's weird but true
( Amberl was stripey and dominated Europe, Fri 2 Apr 2010, 22:43, closed)

No
Even if it is true and non-negative it's still prejudice if it informs your views on people based on group membership.
( Ungersven would support SpankyHanky's patreon, Fri 2 Apr 2010, 2:34, closed)