Which Arrow Resin Series shed should I choose — the 4 x 2 vertical or a walk-in size?

Choose by what you are storing and the space you have. The 4 x 2 ft vertical shed holds about 22 cu. ft. and stands tall, so rakes, shovels, trimmers and a folded chair store upright against a wall, on a patio or a balcony; it comes in Dark Ash and Ash White. The walk-in sizes — 6 x 4, 6 x 8 and 8 x 10 ft — have swing doors you walk through, a window for daylight and a high vented gable roof, in Brown Ash and Ash Grey. All of them use the same resin panels, include the same floor and have the same pad-lockable handles, so the choice is about footprint, not about weather resistance.