carb syncing...nipple size?

[1990dtgl98]

Hey guys, my wife has a 2002 GS500, and I recently did some shim swaps since the exhaust side was a little tight for me. Some of the driving issues were fixed...but not all. Now I want to build a manometer to sync the carbs, however, it's in a slightly tight position to get to. I'm running out for supplies now, so can anyone tell me what size inside diameter tubing I need to fit over the vacuum nipples?

That's all I need. Thanks everyone!