[abcd_f]

Letters were 8 mm.

> To demonstrate the system’s capabilities, the team created a series of 8-mm-wide targets, each made from a reflective material and imprinted with a letter.

[hannasanarion]

I checked the paper, by "8mm wide" they mean that the letters were 8mm tall, which is a 22pt font (name-tag size), for those curious.