This is about the smallest one I've seen around. You need to somehow anchor it to the ground, though, or it can blow away. They have one set up at our local Menards.

Keter Manor 4x3 shed

Here's some lower ones, but they are all resin-types. I've found that the ones with the flip-up lid have a lid that can warp over time and critters like to get in (mice, wasps, etc.)

Lower storage

Anything even tinier? LIke 3 x 3 that can mount on a wall maybe?

Thanks!

If you search on pinterest for "wall mounted garden shed" you'll come across some ideas that have build plans...but as far a commercially available, I'm not sure if there are any.