Towing with a Toyota Sequoia: wheel size question

I have not had issues with 225-70-19.5 lrG tires on my mdt. I would agree with grit dog, get into to 50 or lower on some time sizes. Locally some cement rigs are using 35-40 series ties in the front axle. This have 6-8" of sidewall too. No issues from what I've heard or seen.
The real issue IMHO with LOW profile tires will be the actual sidewall height per say. Have 2", you will have issues. Have 5-6+", you have sidewall height to allow flex and not hit the rim. I would also assume, higher psi in a given tire size, would also not allow the sidewall to flex, such that the rim hits a curb, pothole etc to damage it.
As I stated before, plus and minus's to the two tire sizes I mentioned above, add in an 18-20" rim equals, 265mm in width, same 31.5" overall diam. Just lower sidwall% height, those could have a higher potential for rim damage, be it on the street, or off roading.
Marty

Sequoia owner here. Like others have said, you may want to consider LT tires. I bought a set of 18" take-off wheels (Tundra 5 lug wheels match exactly) for putting a set of LT-C tires on. I've only towed with LT tires and am happy with the experience. I'm not sure if LT is available in the 275/55/20 size, you'd have to check tirerack.com or other online tire sites.

A little more info: I've towed my 5.5K travel trailer (12.5K combined at the scale) with my 2011 Sequoia for 8 seasons, 50 or so round trips through the Colorado mountains. Performance is excellent up and down the hills.

Thanks for the input, everyone, very helpful. The OEM tire size is 275/55R20 (sorry for not including that in my original post), and I'm gathering that the general consensus is that a new wheel size is not necessary, so I'm looking at replacing the current tires with the Michelin-LTX M/S2. I guess my younger brother implied that our Sequoia's big rims were more for looks than towing, but he lives in Tahoe and does a lot of complicated rock crawling. He drives a Ford F350 van customized as a deep-off road rig, and it's all function! We're not planning on doing anything more backcountry than the occasional graded dirt road in summer in a national forest campground, and we're definitely keeping the GVWR below 5000. Thanks again, I'll let you all know what we go with for a TT!

^ You're correct.

You can't downsize the tire/wheel enough to make an appreciable difference in towing performance. For that to happen the tires would have to be so small that the rims would not fit over the brake discs. It would look ridiculous.