Banners need a much bigger code than you'd expect
The same distance-to-size rule that applies to posters applies here, just at a bigger scale: a QR code should be roughly one-tenth the distance it needs to be scanned from. A banner meant to be scanned by someone standing 2-3 meters (6-10 feet) away — typical for a pull-up banner at a booth entrance — needs a code at least 30cm (12 inches) square, and a backdrop meant to be scanned from further across the aisle needs to be bigger still.
That's a big jump from the code most people picture, since the only QR codes most of us handle day to day are business-card or receipt sized. If a code looks "too big" on your banner proof at normal viewing distance on a screen, it's very likely the right size once it's printed and hung eight feet up at a trade show.
Standing out on a crowded show floor
Trade show floors are visually loud — competing booths, sponsor logos, lighting rigs, and foot traffic all fighting for the same attention your banner is trying to catch. A QR code needs to hold its own against that, which means strong color contrast against the banner background, a generous quiet zone around it so nearby text or graphics don't crowd it, and a spot on the banner that isn't likely to get blocked by a pop-up display, a plant, or a cluster of people standing at the booth.
Resist the temptation to bury the code inside a busy background photo or a gradient section of the banner design just because it looks cohesive — cohesive and scannable aren't the same goal, and at booth scale, scannable should win.
What to link to
Attendees walking a show floor are moving fast and making quick decisions, so send the code somewhere they can act on immediately: a lead-capture form, a digital catalog or portfolio, or a "save our contact info" vCard link all work better than a generic homepage they then have to navigate from a phone. Pick the one destination that matches what you actually want someone to do after they walk away from your booth.
If you're also working through the print production side of a large banner — material choice, DPI, finishing — that's a broader topic than QR sizing alone, and QRSen has a dedicated page on large-format printing considerations worth reading alongside this one.