This notebook uses Rite in the Rain's specially treated, fully recyclable paper with their "universal" pattern, which is a wonderful combination of lined and gridded, making sketches and writing equally easy and readable.
This is called their "outdoor journal". Its size splits the difference between the pocket size and the other difference-splitting size. This belongs in the breast pocket of a person in a ghillie suit, but maybe you could use it, too.
Because it's their all-weather paper, you won't have to worry about sweat or rain being any trouble.