4 × 4 is the smallest Kakuro board and the right place to learn the rules. Runs are two to four cells long, so most clues have only a handful of possible digit combinations — several are "magic blocks" like 3-in-two (always 1+2) and 17-in-two (always 8+9) that you can fill almost on sight.
Because the boards are small, a full solve takes just a few minutes, which makes this size ideal for practicing the core habit of Kakuro: check what a clue allows across, check what the crossing clue allows down, and keep only the digits that fit both. The combination reference lists every clue's possibilities.
New to the game entirely? Start with the how-to-play guide, then come back here. When 4 × 4 feels comfortable, 6 × 6 boards add longer runs without a big jump in difficulty.