I have this exact x-pen (I even ordered from Chewy.com, they’re the best) and I love it!
I’d say the bigger the better. English lops may not love to jump very high as a trend, but you never know what your bun will like to do — or discover they CAN do!! c; Plus, English lops are pretty big, which means they can stand, and therefore jump, fairly high. Theodore, for example, never liked so much as hopping on top of a box that he could easily make, but when I got him a playpen that was too short (19″), banking on his lacking an affinity for jumping, he cleared it with ease.
Theodore is a medium-sized Dutch, and for him I have the 42″ high pen as his home pen — I ended up getting a second playpen that was taller, at 29″, and that works great for him as a playpen. The 42″ is a little overkill for him, and I got the pen because I was originally planning on adopting a much larger bunny, but I gotta say, it really puts my mind at ease knowing that there is NO way he’s going to jump over the top while I’m away.
If you don’t want to go that high, I’d say 36″ is your very bare minimum. Honestly, though, if I really were in your shoes, I’d still get the 42″. It’s a shame to buy something you think will work only to have your bun prove you wrong, and then have to spend more money on something you could’ve gotten in the first place.
I also recommend attaching (I use clothes pins, cheap wooden ones from the dollar store) a cotton sheet to the top or half of the top of the pen. This helps discourage jumping.
Good luck!!