These aren't exactly what you're looking for but they might help.
First up we've got a series of close-up photos of the replica targeting scanner built for the TNG episode "Relics", which was ultimately never shown onscreen: http://www.startrekpropauthority.com/2010/09/tos-style-helm-targeting-scanner-from.html
And here is a set of blueprints for the TOS Enterprise bridge by Michael McMaster. Sheet #4 shows the targeting scanner and feature a scale, so a little creative work might let you figure out the dimensions: https://www.cygnus-x1.net/links/lcars/mcmaster-uss-enterprise-bridge-blueprints.php
