I'm not sure if anyone else has encountered troubles when creating a Submarine in Barotrauma's editor when a sub design's size sometimes exceeds the randomly generated underwater cavern spaces in a level. There's no measurement system in the editor far as I know, (last design I decided to use the heavy wall object as a measuring of my sub's total dimensions, roughly 16 pieces long and 4 and a half pieces tall).

Does the size of it affect how the auto-pilot functions ? As my last creation had difficulty navigating through on auto despite all systems being properly powered, even in the largely spaced horizontal areas, it would keep at a steady crawl towards the destination. Or is it based on the ballast size or setup of the Sub as well or what is configured in the navigation terminal's 'default ballast level' in the editor ?

Cause I do enjoy creating a few designs in the editor to test out, but it's hard to tell what could or is exceeding the limits of the level's spaces to pilot in, usually cause I cannot check the sub until it's properly wired up and configured to be sea-worthy.

There's no measurement system in the editor far as I know

Submarines consist of square blocks, they're the size of a smallest possible gap/hull you can draw, or a length of a shortest wall (not width since even non-heavy right/left/top wall have width of 2 blocks). E.g. hatch is 8 blocks wide and 3 blocks high, doors are 13 blocks high and 3 blocks wide, lamps are 2x1 blocks etc.

As for Ideal size I'm not sure, but I have a feeling that in latest versions map generator generates map so that it would accommodate the submarine. Recently I only encounter tight passages on small subs while never getting them on bigger ones. The last time I was stuck was on Kullervo Carrier in 0.6-something which was almost a year ago. I rarely play big subs though so I might be mistaken.

Auto is slow everywhere, it depends not on the sub but on the shape of the rock corridor your sub is in. If you watch autopilot in action you'll see that unless you're in open sea autopilot is jerking directions all around resulting in acceleration being changed several times per second in every direction and averaging to almost zero.