Which dictionary should I use?

There are ArUco markers with 3 x 3, 4 x 4, 5 x 5, 6 x 6 and 7 x 7 chessboard size. The smaller the chessboard size the smaller the physical size can be of the marker and on the other hand the larger the change for false positive matches (e.g. some natural/artificial objects are very similar to a marker). To find the optimal physical size and chessboard size for your case please use the GSD and ArUco marker size calculator.