There are special considerations for the heavier rider to think about when picking out an Icelandic Horse. The information on Weight-Carrying Ability is here.

Pacey horses are not a good choice for heavier riders since it makes it harder for the horses to lift their back. The additional weight makes them more pacey. A horse more on the trotty side would be better.

Conformation of the horse will be something to consider, also. You'll try to get a horse with wide loins and also a wide barrel. Once you have those things, you'll want to check to see where the legs are set on.

Here are a few pictures of choices you might encounter:

A horse with legs set on like the legs at each corner of a table gives good stability. A horse with legs close together is not able to balance as well, especially under the weight of a heavier rider.

Take a look at the legs in the first picture, compared to the legs in the next picture.