TACTICS CARDS DRAWING TOO MANY CARDS/HAND SIZE

If I am using an ability that makes me draw more cards than my maximum hand size, when do I discard back down to 5 cards?

Always complete the entire effect granting the card draw first, do not stop mid effect. As soon as that effect is fully completed you check to see if you are over maximum hand size. For example with letters you would draw both cards and place the condition token, then discard if still above maximum hand size.