There is a diagram with measurements on it in the new WA/FITA Book 2 - Section 7.2.1 which gives target numbers and flag sizes for indoor and outdoor. (
WORLD ARCHERY click Rules, Constitution and Rules and look in Book 2 - Events)
Outdoor number are 30cm high (black on yellow and yellow on black starting with 1 black on yellow background),
flags are 25-30cm triangular (yellow and light red) on poles 40cm above or below the face.
Indoor they are 15cm (black and yellow alternating starting with a black one on yellow), no flags.
As to the shooting line numbers:-
7.1.1.7. There shall be a mark on the shooting line directly opposite each target butt. There shall also be a
number corresponding to that target butt between 1 and 2m in front of the shooting line. If two or
more athletes are shooting at the same target butt at the same time, the shooting positions shall be
marked on the shooting line.
Does not specify how big the numbers are though.
Rip