8 × 8 is the classic Kakuro size — it is the format our daily puzzle uses, and the sweet spot between a quick fill-in and a long sit-down solve. Expect a real mix of run lengths, clue sums that need combinatorial thinking, and boards where the opening move is not obvious.
Two techniques carry this size. First, minimum/maximum bounding: if a clue's other cells can contribute at most so much, the remaining cell has a floor and a ceiling. Second, combination elimination: as digits land, many of a clue's candidate combinations die, and the survivors often agree on a digit — that digit is safe to place.
The strategy notes on the combinations page walk through both, with the full lookup tables. For a longer challenge, try 9 × 9.