I can't find that product either, but I'm going to guess that it's a wall-bracket for storing a ladder, something like this or this
The length of the screw you need depends on the thickness of what you're screwing into. You don't want the point coming out the other side and gashing you every time you walk past it.
If it's a wooden shed, you need to screw into the wooden framework (not the cladding material). In a cheap modern shed that might only be 30x30mm, so you could only use a 30mm screw (drill a pilot hole first or it will probably split)
If it has a thicker frame, you can use a longer screw, but always shorter than the thickness of the studs.
If it's a brick shed, use a 40mm brown plasplug and a 40mm screw.
If it's a sectional concrete building, come back because they are not very suitable for screwing into, and you may crack the panel.
If it's a metal shed you need a different approach.