Rite in the Rain All Weather Pocket Notebook (135)
This is the smallest in the range at 7.5 cm (3 inches) wide by 12.5 cm (5 inches high) - perfect for shirt pockets. As well as the note sheets, the back also has measuring scales printed on to help you with field measurements of small items.
If you want a larger version for your hip pocket then take a look here.
More information on Rite In The Rain can be found here. And don't forget that these products were just perfectly made to go together with Space Pens!