Why are guages so large?
 

Particularly the depth guages...is it so they can be easily read when all hell is breaking loose in the sub? :o
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http://users4.ev1.net/%7Etaragem/sh3_u100.jpg

And also because everything behind those gauges was mechanic...

Yes basically since it was a pressure operated mechanical device it was easier to acheive greater accuracy or resolution with bigger devices.

Also the guage could be read from nearly anywhere on the bridge at a glance.

Yep. Large gauges translate into greater precision, but I would think viewability was the main reason.