Standoff's load-bearing capacity and common dimensions

How much weight do they hold?
Each 1" standoff can hold 50lbs of weight, so a typical 4 corner application could hold 200lbs of weight (depending on the wall structure and anchors). Drywall installations require proper anchors depending on weight of the piece.
Common Standoff Dimensions
hexagonal male-female aluminum standoffs are intended specifically for use with the Raspberry Pi. They keep parts spaced 11 mm apart, which is just the right height for mounting expansion boards with standard-size female headers, and they have 6mm-long M2.5 threads that are just the right size for the Raspberry Pi's mounting holes.

aluminum hexagonal standoffs are just the right size for mounting expansion PCBs or HATs with standard-size female headers on a Raspberry Pi. The body of each male-female standoff is 11 mm long by 4 mm wide. The male threads extend another 6 mm past the standoff body, and the female threads extend at least 6.35 mm into the body. You can easily screw multiple units together to make longer standoffs. The threads are M2.5 on both ends, since this matches the size of the Raspberry Pi mounting holes.

Most screws in a computer are UTS 6-32 screws. 6-32 screws are used for your power supply, 3.5" hard drives, securing expansion cards, and the case panels.
The other common screw type is the metric M3×0.5 screw, which are used for mounting optical drives, floppy drives, and the motherboard.
The thread size for an NVMe drive is usually M2×0.4, although many drives are secured with a tool-free design nowadays, typically a plastic lever which rotates.
To make it harder, there are also computer cases that ship with motherboard standoffs that accept imperial screws, not M3.
It's amusing that both imperial and metric screws are used in each computer, and rather annoying that these screws are fairly similar in size (although not in threading). This means that an M3 screw will fit in a 6-32 hole, and will even seem to screw in, but it won't stay in place well. A 6-32 screw will go in an M3 hole if you try hard enoug