I searched for a screw sizer for my wood countersunk screws, and all those I found required two steps to measure, one step to measure the diamater and another to tell the length.

I wanted a tool that will allow measuring both dimensions in one operation and show both dimensions at the same time. This is what I ended up with.

Print instructions: Make sure to print with the flat side at the bottom. The files might have come out on the opposite side and require turning in the slicer. I used standard settings on Orca/Bambu slicer, 0.2mm layer height, there's no need for support. The only setting I changed to get better text was to disable “Only one wall on top surface” (in the Quality settings).

To get the text to show well I painted the top layers, those with the text, in a different color. 

Assembly instructions: You will need at least two (can use four if want) M3x12 countersink screws to connect the two parts. 

Usage instructions: Just insert the screw at a position it can enter easily, it can be at the widest opening, then slide it down until it can't progress any further down, finally push into the contersink. On one axis you will see the length and on the other the diameter. This entire description is practically a single quick and easy operation.

Disclaimers:

1. The tool supports only sizes for the wood-screws I needed, (length and diameter) but it should be easy to extend to other sizes as well. I don't know how well it would work for other type of screws, so don't judge the model on that. This is why I included STEP file, for easy modeling further if you want.
2. It's optimized for contersunk screws, might work for none contersunk but didn't try and it may not be accurate for those because the screws will slip into the screw head hole.