Would longer heat exchangers (your current project displays 10ft of 3/16" in 3/8") or larger diameter tubing or both increase performance?
Yes longer would be better, so long as you don't take it to an extreme and end up with too much pressure drop on the suction side (higher evaporator suction pressure due to pressure drop would = warmer evaporating temperature). I would think you could get away with doubling it.

Yes larger tubing could also be used to make up for pressure drop of longer exchangers.