I prefer thickness, gives the member a kind of a weight. I don't look for them to be long, just... heavy perhaps.
Also, a nice, juicy head. The bell-end is important and needs to also have thickness and presence to it.
Same for me with testicals, I like a nice sack of balls you can almost see - especially if its a little tight so you can see how large they are - and what they're filled with ^^
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