Length length length! Must be at least 6 inches or more to really excite me, 7 or more to get me foaming at the mouth lol.
Must also be circumcised.
And the size of the cock head. A nice, large, round cock head at the end of that nice long, hard, pole... mmmm! Girth really doesn't matter to me, just length and the size of the cock head in proportion to the length itself. I'm probably in the minority but there is actually something I find hot about a thin, long dick. Girth is hot af too, but hardness man... seems to me the thinner dicks get ROCK hard. Which brings me to the next point...
To a lesser degree, hardness. You can tell just by looking at a dick how hard it is. When a dick is hard, like balls to the wall rock hard... yes please!
Here's a few pics showing what I consider to be perfect in a dick. Also just noting, this is only speaking from excitement. I wouldn't have a problem with a smaller guy who doesn't meet these "ideal" "excitement" stats. This is just what I find ideal and what really gets me off. We all come in different shapes and sizes and I'd have no problem with another guy having a small one too. 3 inches or 9 inches, at the end of the day dick is dick and its how we use them that makes them so fun. Making one cum is the funnest part of all and size does not matter to that affect.