we went to ocean shores, washington a couple years ago and they had trail rides on the beach. i had a solid horse and my dh had a belgian. i'm not sure if it's purebred or cross. he was a big solid horse though. the trail ride was mostly walking and a bit of trotting, on the beach. pretty laid back ride. my dh weighed maybe 230, 240. i would think they have horses that can carry you.
i don't remember the name of that particular stable. they trailered the horses to the beach and tethered them, so, we just went up and paid to go. but, it was in ocean shores washington.
i have visited a number of trail ride stables over the years and i think almost all of them have had a drafty or two in their herd. and you know everyone is getting fatter, so, a smart stable will try to accomodate.
not everyone is a fit little athlete.
here's a facebook page for the plus size rider. maybe they know some stables??
Gailforce -- Another old lady rediscovering her inner cowgirl.