Haven't a clue
 

What is it? I found this while demolishing an old building and it was interesting to me..but I have no clue as to what it is.

Looks like an old fashioned loom for weaving

How big is it?

I've seen dish racks built similar to that.

I'd bet you're right, though I haven't seen one before with legs that rest in the sink. Most of the old drying racks I've seen rest across the top of the sink.

Tapestry loom

Nope, look closer, the OP's find has no moving parts like a loom. Reasonable guess at a first glance though.
My first thought was a classical guitarist's footrest, until I saw it wasn't adjustable.

Correct, it is not adjustable, but it does fold down nearly flat

Folded image

I still think Blondsense got it right. Is the "grate" a size that will hold dinner plates? Some of the old sink-top drying racks I've seen also fold for storage.

Here's a photo with a standard dinner plate...not suggesting that's what it is for, but it's a way to show the size.:yes:

Folding beach chair?

Ok, not a drying rack. I was thinking of the feet resting in the sink and the grate horizontal on the edge of the sink, but the plates would obviously fall through.

This has been nagging at me all afternoon, dammit, and I finally figured out where I've seen something of similar design.....I now think it's a staircase plant stand. Or, since it's portable, maybe a staircase bucket holder.

hahah you're funny. Wouldn't the grates make the base of the pot unstable

Not at all. We're looking at a wide flat base resting on several supports in the same horizontal plane. How is that unstable? Maybe you're looking at this thing wrong. The dowels with rubber caps are the vertical legs, those rest on the lower step. The "grate" rests on the step above. The angled outer dowels of the "grate" section act as safety rails, and the pot or bucket sits on the horizontal dowels in between.

I meant you're funny because you were obsessing over it. Its all good. :)
So even if it does fit snugly on a stair, don't you think the pot could still be a bit wobbley. The pot would have to be just the right size...too big and the rails are in the way, too small and an edge may not line up properly. What is a staircase bucket holder. Why would you leave your bucket on the stairs :laughing: