I really like the Columbia bonehead fishing shirts. They are very lightweight cotton and have a mesh cape over the shoulders that allows ventilation. Plus they have a ton of pockets on the front. And they are available in 3xl and sometimes 4xl.
They are cotton, so I wouldn't recommend them for anything except hiking on hot, dry days - but for that purpose they are great.
3 and 4XL for the zaphtic survivalist, heh.