A bunch of spinach usually weighs something like 250-500 grams. My best guess is that something like half that is the stems, which can easily be as long as the leaves. So very roughly, I'd say a bunch is 125-250 grams. Alternately, if you want to go by volume (e.g. if your store has bulk loose baby spinach), think of a bunch as something like a head of lettuce, except with the bottom third to half mostly stems.
Full-size spinach also definitely gets a lot better than 4x6 cm leaves, so you may still be using somewhat more tender baby spinach, but that of course won't cause you any problems. (The stems on baby spinach are tender enough to eat, especially when blending, so you can use everything in your bags.)
In any case, smoothies are really forgiving - not only do you not need a lot of precision, but you can just add and blend until the color and flavor look how you want!