Any horse and most ponies and large dogs are stronger than a human (plus as my DH used to say, they have four wheel drive). I am pretty sure they all know it as well, so you probably didn't teach your Belgian anything she didn't already know, but it's the kind of habit that could be a real pain in the pettuty. The problem, as you say, is she's pushy and disrespectful. Everything you do with her has to be a lesson in respect, not just whether you have a round pen or not. When you lead her, she has to learn to walk nicely. When you come to a gate, she should let you go through first, or wait till you tell her she can go through, when you do her feet, is  a chance to teach manners etc.. I suppose the idea of the round pen is supposed to teach that, but millions of horses had to learn it without a round pen as well, that's just one way to do it. And then there's those horses that think outside the box so to speak and just might try to climb the round pen. So build yours accordingly for her size if that's what you are going to rely on. Meanwhile i would want some up close and personal training with her on everything else. Don't give her more liberty than she can handle, so work her in a relatively confined area till you do the round pen thing.

Also, if you get her leading nicely in a smaller paddock, then lead her in the field, don't let her forge or piss around. If you are going to turn her out in the field, lead her out, make her walk nice, then give her permission to go, when you are good and ready. If she does that you can lead her on a little longer lead, and eventually will be "lunging" her, but not really lunging her full out on a long line right off the bat.