Size and Thickness Comparison
As to sizes, try both sizes. The smaller disk was originally
designed for kids, but they are very nice for everyone, not just smaller hands.
The slots are closer to one another, so the hand does not haver to travel as far. However the EZ-Bob get a bit more entangled as they are closer to one another. The larger disk is recommended for any patterns with more than 8 strands. In addition, if you use a pattern where you need to read the numbers, they are easier to read on the bigger disk.
As to thickness, if you find that the thinner disk does not hold your tension tight enough, try the thicker ones. Keeping newer disks for thinner cords and using older ones for thicker cord works well tool.