Does anyone know what size drilled stopper I should order for this?
#2 drilled stopper. I would order a few. Doesn't hurt to have extras.
I've also used mine to fill various size/style growlers too. Here's a lit of stoppers I've used.
#2 - for standard 12-22oz bottles, also includes the some swing tops
#6 - for standard 1/2 gallon glass growlers. They work on my full gallon jugs too, but I've not used one as a growler.
Small universal stopper (what comes with the plastic Big Mouth Bubblers), I've used these with the growlers that look like small kegs.
Large universal stopper (the ones with the Better Bottles). These will fit the 2L swing top glass growlers.
I thought I would throw that out there in case you already had any of these stoppers. The only one I really had to add was the #6 as the others were included with the fermenters. I use the Better Bottle one the most since I like the swing top 2L growlers the best.