0.5 inch screws are the most versatile. They can be used to attach pieces of metal to each other, attach bearing blocks, attach multiple pieces of metal, and overall are more versatile than the 0.25 inch screws which cant be used for attaching bearing blocks. In other words, they double as 0.25 and 0.375 inch screws.
1 inch screws are good for attaching 2 pieces of metal with a gap between them.
Screw _ metal _ spacer _ metal _ screw.
They can also be used with spacers to create protrusions from the surface of the metal, which is good for attaching rubber bands or other things.
1.5 inch screws are good for short screw joints, or attaching things across even larger gaps. Also good for boxing, as they can go across a c channel’s width.
2 inch screws are good for larger screw joints. I used one for my tilter this season, and it held up pretty well. Also good for attaching things across even larger distances, such as the 2 sides of a tilter.
You can go with other sizes, but from my experience, the other sizes haven’t been as useful. Only get other sizes if you want to do less screwing in, as these 4 sizes will have everything covered.
In cases where you need a smaller screw to fit in a tight space, you can allways just cut them.