First time I bought a slim shelf, I didn't measure. Guess what? It wasn't slim. It was a normal shelf squeezed between a doorframe and a radiator. Looked like it was holding its breath. Learned that lesson the hard way.
Start with the wall. Not the shelf. Measure the depth from the wall to whatever's in front—the door swing, the baseboard, the evil radiator. If you've got 8 inches, you're stuck with shelves that are 6 or 7 inches deep. That's the slim shelf. Anything deeper and the thing sticks out like a sore thumb.
Material matters more than you think. Particleboard with a fake wood sticker? It'll bow the first week you load it with books. I've seen it. Plywood is better, but real wood is the only thing that won't sag under a stack of hardcovers. And metal? Steel frame with wire shelves—great, but the thin ones bend if you lean on them. Don't lean.
Here's what I did: bought a 7-inch deep steel and wood job for a hallway nook. 12 inches wide, just enough for keys, a small plant, and a paperback. Looks fine. The problem? The floor wasn't level. The feet didn't adjust. So I stacked felt pads under one side. Works, but feels dodgy.
You want adjustable feet. Non-negotiable. And a wall anchor—slim shelves are top-heavy with anything tall on them. I used a cheap bracket kit, drilled into studs, and it held a ceramic vase without drama.
Now the catch. If you need more than one shelf, measure the wall height first. A 36-inch tall slim shelf looks squat next to a 6-foot doorway. Mine's 30 inches—perfect for that nook, but I could've gone higher if I'd bothered to look up.
You can also use a slim shelf as a nightstand, like this one I messed up. Found that out after the fact.
If you have a full foot or more of depth, skip the slim stuff. Go with a normal shelf—more stable, uses space better, and it won't look like a compromise. Also, if you're loading it with heavy tools or full wine bottles, forget it. Slim shelves are for light duty.