The preKSP1-11 patch should be fine on KSP1-11 and above as it's predicated on not having a part that is in 1.11, and should do nothing in 1.11 and above (it was ignored when I checked the MM logs during testing). This is based on the stuff that is in the thread below and currently in the Near Future mods (https://github.com/post-kerbin-mining-corporation/NearFutureElectrical/blob/master/GameData/NearFutureElectrical/Patches/NFElectrical1-10.cfg), but please feel free to leave it out, as I said I don't have and old copy of KSP to test against.
I based the cargo size on the pico ports being 1/4th the weight of a Clamp-O-Tron Jr (0.005t to 0.020t) so, given the same density, they be 1/4 the volume (35l to 140l). A quick idiot check based on the pico port being 0.3125m in diameter (half of 0.625m) gives a 35l box that is approximately 0.3584m high, so it seems to be in the right ballpark.
Edit:
Having written that down and thought about it for a couple of hours, I'm not so sure my logic is sound. I assumed that the density would be the same as it would be made of the same material but now I think about it, it's the overall density of the part the matters which can change. A naive scaling of the Clamp-O-Tron Jr gives a volume of 17.5l, so perhaps 20l would be better than 35l. This does mean that a Kerbal could go EVA with a EVA jetpack and a PicoPort to fit, which might actually be better idea.
My original plan was to avoid this as I felt that having to get a rescue craft, a stranded Mun lander and an engineer all within EVA construction range was a reasonable punishment for leaving most of my hero Kerbals in very low Mun orbit with no fuel because I cant read DeltaV chart.