The dimension of the moldings are the same as the staircase railing to make it fit in. Larger or smaller would do just fine as well. It´s just a matter of taste
When choosing moldings following the staircase and the ceiling in the hallway I wanted them to be seen and not blend in and form a sort of pattern. That turned out especially well in the ceiling around the attic hatch ( top picture) Really pleased with that detail.
Wishing you a wonderful weekend. Enjoy :)