I agree with looking for an older stockier horse. I ride a 30 year old horse and she acts like she's 10 lol. Never spooks, never balks at anything, and teaches me more about riding then any human ever could :). If I were you I'd maybe look for a draft cross. A draft horse for trail riding can be a pain, trust me we have 2. Their size makes it hard on trails sometimes. And draft horses eat A LOT lol.
Here are two examples of horses we have that would work for a heavier rider:
George is a Scottish Highland Pony. Lol don't let the word pony trick you, this horse is HUGE lol, not tall, but build like a tank. He could handle a heavy rider with no problem and is great on trails.
Ruby is a draft cross, were not sure what exactly she was a rescue. She's also built very tough and stocky, but still very pretty and a good size, not too big not too small.
This is Peggy, one of the drafts we take on trails. She does awesome, is an old horse and is dead broke. A horse like her would be perfect for a new rider. HOWEVER, it can be hard, and expensive to find a good saddle for a draft, and everything you get for them has to be extra big, read extra expensive.
I'm not an expert on this lol, just talking from what I've delt with personally :)