I'm a beginner myself, so take this for what it's worth... Unless you're going to set the knot just low enough so the plug doesn't show, the knot diameter should be a bit smaller than the opening, because the knot is funnel shaped when it's bloomed out, and the deeper you set the knot, the bigger the opening needs to be. In the three or four brushes I've done, the opening is a couple of millimeters bigger than the knot.
That said, the knot size you choose should take into account how big a knot you prefer. I like small knots. 18 is my fave. If the opening is, say, 20mm, and I like 18mm knots, that's a great fit for me. I restored a couple of Ever Readys that had a 16 or 17mm opening, and I widened it to 20mm to accommodate my 18mm knot. But if the knot size you like is 20 or 22mm, and the opening is 20mm, go with a 20 or 22 and widen the opening accordingly. I'm working on a handle now that has a 20mm opening, but an 18mm knot just looked too small, so I'm going with a 20mm Whipped Dog knot and have enlarged the opening to 22mm.
Batman - Usually you want to order a size down. Plug sizes vary from knot to knot, so if you order the exact size it may or may not require you to bore the hole out more. If you have the tools, then no worries. If you dont, then its a hassle.
I tend to always order a plug size lower then what the plug measurement is. (22mm hole, then i'll order a 20mm knot.) And that allows me to set it at a short scrubby loft that I enjoy.