Just about every screen for use at the input of a downspout has a very fine mesh, perhaps blocking debris as small as 1/8", even though such small stuff would pass harmlessly through a typical 2x3" downspout. This causes the screen to clog, preventing the gutters from draining and causing them to overflow.
It seems a better design would be something like in kitchen sink drains use: a large cross that blocks only the stuff big enough to potentially cause a downstream blockage.
Thanks for your comments.
Ray