You must take into account the direction of tension when choosing the size of an eye..
All eyes are far stronger when tension is straight away from the threaded shank then when at 90° to it.
Then if the choice is for a tight space you may have to use a smaller one then you otherwise would.
With this in mind.
Generally speaking, other specifics may change a choice for a certain application.
I use-
1/4" Eye's on 3" Airframes.
5/16" Eye's on 4" Airframes.
3/8" Eye's on 6 & 8" Airframes.
Years ago I used smaller, but if the application will allow for a sideways tension, worse in a shock, then go slightly heavier.
3" Airframe with a 54mm mount, my attachment point on the top centering ring there isn't room for a 1/4" Eye,
I use a 3/16" eye with no fear, you cant have any sideways tension..
This is exactly what must be understood.
If the tension is straight away, then the threaded steel eye with steel washers spreading the load as much as possible,
the steel is stronger than whatever it's mounted to. This point isn't a guarantee, but will prove correct 98 or more times out of 100..
Teddy