I've used a label maker from work to make up new lettering .
The smallest label tape is .250 inches wide, about 6 mm .
You can have multiple lines on the tape, so it resizes the dimension of the lettering, so you can get the size you need .
The tape I used, was clear with white lettering .
Other method I've seen used, is small letter decals, I guess it's used with computers, they come on small squares just a bit larger than the letters .
You get a sheet with 10-20 of each letter and numbers .