BX2680? Specifications are here:
https://www.tractordata.com/farm-tractors/009/1/1/9114-kubota-bx2680.html
Presuming you're US-based so you like old school measures, that gives 95 inches long, 45 inches wide, 1,500lbs. That's without loader or tiller, and without mower.
The mower is probably 60 inch / 5 foot. The boot on the side adds a bit, you can pull it up, but you would want 6 foot wide probably.
Length-wise 95 inches is 9 foot. Loader plus tiller, you'll be another 4 foot minimum.
So.....14 foot by 6 foot wide is your minimum I think. Others will know what's common in the US, but I think this would be a common size.
You'll want the BX to be able to pull it, so don't go too big. Also, presumably he'll want the BX to be able to fill the trailer. Watch out for height, the BX doesn't have a lot of height on the front end loader, you want to be able to tip over the side into the trailer. You probably don't want super high sides on the trailer. If you find the right trailer and it's a bit tall, he can always dump a few loads of dirt by the side, so he can drive up onto the dirt for a bit more height, but it's a hassle. Realistically a trailer too tall for the BX to tip into will also be way to heavy for the BX to pull when it's full.
How will the trailer dump? You can get ones you hand pump the hydraulics up (hard work), ones with an electric over hydraulic pump, and ones that go onto the tractor hydraulics to tip it. The last option is best, but I'm guessing he won't have rear hydraulic outputs to drive it - so factor in buying him a set of those as well to make him super happy.
Having said that, if the plan is for the BX to fill it, then he'll pull the trailer with his truck to dump it, then you'll want an electric tipper ideally. The truck won't have hydraulic outputs.
Also consider what truck he's pulling it with. A BX isn't super heavy so most trucks will do it. But you need to know if he's gonna want a bumper pull or a gooseneck.
Last point. I love surprises. But I also love picking my own stuff out. Given a choice, I'd probably prefer be involved in picking the trailer than getting the surprise. He'll probably never tell you it wasn't the perfect trailer, but if he's anything like me he knows what trailer he wants, and getting a trailer that isn't the one he wanted is not quite as good as the exact one he's been driving past in the shop and wanting for a year or more. Does he happen to have brochures or websites he's visiting or something that would tip you off what brand/spec he is keen on? Or a friend that he talks to about it that would know exactly what he's looking at? Hell, would you be accidentally driving past a trailer shop and pop in to look and kill time with him one day....and take notes on what he talks to the sales guy about?