Hi,
lengths of 1", 25mm or 20mm box go on two brackets high up on the right hand wall of the workshop. I'll have maybe a dozen full length up there at any one time and I tend to cut it at 10 feet, so the 'off cut' is 9' 10" or so..
Bigger box, 40mm - 50mm plus some full length of flat go under the two benches on the left hand side. I have some market stall 'table-ends' that these pieces lay on and to keep them up near the underside of the bench top.
That allows smaller off-cuts around 2 feet or less to go under the bench on a shelf, separated into types, box, angle, pipe, etc. Anything in between these lengths, up to around 6 - 7 feet, gets proped up in a corner.
Sheets are a bit tricky but I have a few odds and ends under the benches on the floor, plus some stacked against the wall behind the bench and some more laid up against a tool cabinet.
Finally I have some real heavy angle, flat and bar around 10 - 14 feet long and the only place that'll hold the weight is the floor. It means I have to clean it up a bit before using it, but it's the best I can do.
I guess I'm lucky that the workshop doesn't get too damp, but a D/A sander sorts it fairly quickly if there's any rust on something that I need.
The real trick is knowing where everything is in the workshop!
Regards
Peter