1mm at 1.36 km works out to about 150 milliarcsec (mas), if you're used to those units from astronomy contexts.

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.

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.

i'm a bit confused when they don't measure things in olympic pools and bananas for scale