I'm an older guy and I've loved sucking dog cocks for decades. (I have NO desire to do anything with a human guy at all!) I've been with large, medium and small dogs, neutered and intact, young, old, most any description. I like medium sized dogs with a comfortable mouthful of knot.
I once had a fairly big dog who had been neutered as a pup (before I got him) and he had kind of a pencil dick. He loved to be sucked, though, and he'd face-fuck me like there was no tomorrow and tie with my mouth for maybe 10-12 minutes every time. His penis was surprisingly thick and knotty when he was erect! I love the taste of a neutered male dog, even though, as a rule, they don't squirt anything. Some of them will surprise you with how horny and into it they are.
My favorite "flavor" of dog cock is elderly, intact males. They're unlikely to get it any other way and they're grateful for the attention. The older boys often as not aren't very humpy so I have to do most of their work for them. A smallish, older Lab mix would frequently show up at my house and invite himself in for a blowjob. He'd always gift me with a tasty mouthful of doggy juice, then he'd be so happy afterwards that he would be all full of kisses and cuddles. He was rather well endowed for his size and sometimes he'd let me dock with his sheath.
EDIT: typo.