I did photongraving etching for about 10 years. we made magnesium leather stamps, from the simple to very detailed stamps from yoursuplied art work or concept.That was about 20 years ago.
The etched engravings were made on 11pt (.153) and .250 magnesium sheets, up to 30"x40" sheets. The only limits on size were on the customers end.
11pt was etched up to .130 depth, .250 was etched to .220 up to customers demand.
Look up your local photo engraver.I'm in California there are still 2 in my area .