From ESOUI Wiki
If the player is viewing a bag that is considered a "bank," then this function returns the Bag value representing the bank that they're viewing. If the player is viewing the Guild Bank, or if they're not viewing a bank, then this function returns BAG_DELETE.
The player's Craft Bag, house-local storage, and personal bank are all considered "banks."
if IsBankOpen() then local bag = GetBankingBag() local size = GetBagSize(bag) d("The current bank can hold " .. size .. " items.") end