1) Diameter? They should be 3mm.
2) I was told that a screw can go 5mm into the can before shorting the motor out.One of the features I like the most about the CC Neu motors is that they have a cover over the coils to prevent people from using screws too long. What I usually do is screw the screw into the motor until I feel some resistance (turn it in by hand so you don't damage the coils) and then back it out a little and hold it up to my mount to see if it will fit. If not you need to trim the screw or get a smaller screw. I have a toasted Tekno Neu 1515 1.5D sitting right here because I didn't check the length of my screws
EDIT: Oops, I re-read #1 and realized it was the endbell screw. It may be a 1mm screw, either that or a rediculously small standard screw. I tried to put my 1.5mm driver in the end of my 1.5D and it was too big, they're really tiny screws.