For my wife, size isn't necessarily a requirement for play, as we've been in the swinging lifestyle for a long time and she has enjoyed lots of different kinds of play. I can remember a couple of men who she's found handsome, who were respectful and flirtatious and did well with having me present, who turned out not to be packing a magnum size cock. They weren't turned away for it, a good time was had, no regrets, BUT she doesn't think of or refer to them as bulls, and they tend to move on quickly. She's never had to tell somebody he didn't do it for her, I think the decision not to keep it up has been mutual, even when it was fun the first time. The root cause is that there just weren't fireworks, and I think everybody felt that to some extent.
And it's not solely a function of dick size. She's encountered a couple of men who were hung like horses who didn't earn the title of bull either. One was a total teddy bear. Really nice guy, but lacked any of the energy and presence that drives her crazy. He wanted slow, romantic, passionate love making, but she gets plenty of that with me. It's not what she's looking for in a bull, and he wasn't looking for a lady who needs her hair pulled or her ass smacked. The other one was a decent fuck, but didn't really care much about the cuckold dynamic. She enjoyed it, and probably wouldn't rule out playing with him again should their paths cross, but again, he doesn't fit what she's looking for in a "bull."
Not too long ago, there was also guy was making decent headway chatting her up, and lost her interest when he said that what he's looking for is a switch who will Domme and peg him. We wish him luck and think everyone deserves to find what they're looking for, but he's not my wife's idea of a bull.
I guess the stag/vixen crowd use the word bull as well, but for us, it's very specifically someone who fills the single male role in a cuckold dynamic. My wife looks for dominant bulls who want to help her cuckold her husband. Being hung at least noticeably more than I am (and I'm about average) is kind of part of that role, as are stamina, strength, and at least some level of dominance. It's a complete package kind of designation, greater than the sum of its parts.
Despite having experiences we can talk about now, we're not really super high circulation. These experiences have been over many years. Her cukoldress friends think she's picky as hell. She doesn't have time to talk to more than maybe a guy or two at once, and she prefers ones who bring enough chemistry to make it a regular thing, so she'll go long periods without seeing anyone besides me, and often she'll have only one other. That man is her bull. Other men have come and gone, at events like Splash Mocha, or failed attempts at finding someone worthy of becoming her bull. I've always liked the way Dr. Dave put it on his Stag's Stable podcast (paraphrasing): "You don't get to decide you're a bull. 'Bull' is a title a couple can choose to give you. You earn it." For us, that's done by making my wife cum more times than she can count in a single session, making her want to submit to you, while having your way with her, right in front of me, her adoring husband. Maybe you can do that without a big dick, but I mean, your odds taper quickly the smaller you are.