Altium Arrow To Designator
Around 0.15 min line width and 1.5 mm height should be big enough.

There isn't any standard about how to arrange the ref des in the PCB. Just try to make it as useful as you can. That's the reason that there is no point in putting it underneath of components.

As some mentioned, ellipsis (Rx.. Ry) are fine, as well rearranging the ref des in the same way than the physical components in a space nearby.

The key point is thinking about who is going to read/use that silkscreen.

If something is ambiguous, I would delete it. It can cause more problems.