In career mode, procedural parts only has tanks that can be sized to a certain amount. I think you have to unlock the tech tree to be able to have tanks that can be sized to any size, but I don't want to do that, so how do I adjust the .cfg file to make it available right now?
I FOUND THE ANSWER.
You have to go into the config in the mod files, under the folder "upgrades" and then change the configs 'techrequired level' for the unlimited size upgrade to, say engineering 101 or something low level. You can delete all the other upgrade levels, since unlimited is what you want.