As I said you have to use the Modded beta, if you are completely new to steam beta versions of games, then here's what you do. Right click the game, go to properties -> betas -> switch to the Moddable version. Many games have these beta or experimental versions with extra functionality (but they are not completely 100% stable, more bugs are commonly found in these). Make sure you are subscribed the mod and, if I recall correctly, it should either be together with the other tubes or it might be in it a "modded items" category. 'sbeen a long time since I played.

How do you even open/use the mod in the game? I don't have the option of having the different sized tubes.

Make sure you are using the Modded beta, the example mods really do work quite well.
(But a tiny bit of shame on the developers to not include a simple screenshot like this on their workshop files and not providing accessible documentation)