Jim Breen's edict translates 大さじ simply as (American) tablespoon, but I'm reading directions on a green tea package that says 一人大さじ一杯 and it seems like a lot of tea for a single 湯のみ. I know there are special utensils used to measure tea for the tea ceremony, but this is normal green tea and not matcha. Might 大さじ have a different meaning in the context of tea? Teaspoon sounds more reasonable.
For example, weblio.jp gives the following definition for 大さじ:
which I translate as:
A 15 mL measuring spoon. Or, a fairly large spoon (匙) such as a teaspoon.
Now that second definition is nerve-wracking. Does common sense dictate that when measuring tea, you reach for a teaspoon as your 大さじ?