I avoid sitting large dogs, because I want to be able to control dogs and pick them up, in case they’re triggered or there’s say an off-leash dog I need to protect them from or they become sick or injured and are unable to walk. That’s to protect bystanders, me and others as well.
I’m especially careful about dog size when a sit has stairs, because I wouldn’t want to be a solo sitter and there’s an unexpectedly incapacitated dog to carry.
On one sit, the senior, sickly dog had back to back seizures and was incapacitated. I had to rush her to 24/7 urgent care. That required me to carry her down two flights of stairs, across streets and a parking lot late at night to get her into my rental car and into the vet hospital. I was so relieved that I could take all of her weight on my own — there wasn’t anyone around to even ask for help at that time of night.
Years ago, my own rescue dog was stung by a bee and went unconscious — it turned out he’s allergic. I had to carry his dead weight from house to car to vet office. So relieved I could do that quickly on my own, because it turned out he urgently needed to get a shot against the allergic reaction.