Firstly, we have no idea what you have for a faucet/tap. This is critical in determining the correct size of tubing. Now I have a Perlick faucet with a removable spout, so I purchased a custom-made Stainless fitting that threads into the faucet and provides an airtight seal and barbed end. I use a small bit of vinyl over that and then a length of rigid plastic tubing that goes into the bottle/growler, long enough to find the bottom.
I think the best course of action for you is to take a measurement of the height of a growler, ad a few inches, and plan on that being the length. You could also measure the diameter of the spout to determine the correct tubing diameter, either inside, or outside the spout, depending on whichever you feel would make the best seal.