Big thanks to all of you who have posted in this thread, especially Corwyyn for those pics with the measurements! I went with 3 1/2" and 1 1/8".
Yesterday, my set of Hella 500FFs arrived and I spent the better part of the afternoon installing them. Amazon had what I thought was a pretty good deal - $67.99 for the kit including covers:
http://www.amazon.com/gp/product/B001TWD5GY/ref=oh_o00_s00_i00_details
I followed the info on this page to drill out a plug in the firewall to pass the switch wiring through:
http://en.wikibooks.org/wiki/Jeep_Liberty/Electrical/Firewall_Penetration
I mounted the switch in the extreme left lower driver's side panel (facing the cabin) that the fuse box is behind. The wires that passed through the firewall were run up behind the kick panel removed as shown in that wiki page link shown above, and with the panel back on were neatly concealed. There is a metal bracket under the kick panel with a couple of studs with nuts on them. The ground wire from the switch fits perfectly on either one, although you may need to extend Hella's short ground jumper an inch or two.
I tapped into the high beam wire going into the headlight socket on the driver's side, so that when my Hella switch was on, the lamps would light up with the high beams.
2002 INFO: There are 3 wires in the headlight bulb socket. The one in the middle is the ground. The high beam positive wire is pale green with a brown trace. The low beam positive is the other one.
I mounted the relay on the inner fender near the battery because I wanted the run with full power to be as short as possible. I drilled a tiny hole and used a short sheet metal screw from my junk drawer.
If I get a chance to take some pics, I'll post them later. Anyway, thanks again for all the info that made this possible.