Well the classic answer I guess is “It depends.”
A bead is primarily a weight which takes your fly down faster and in a river will keep it at the depth you wish to fish. So -a) the bigger the bead the heavier it is b) tungsten beads are heavier than brass beads.
However this being said coloured beads can be used as a hot spot and because they are well forward can make your pattern move up and down during the retrieve.
The chart below is a rough guide to get you started.
| Bead Size | 5.5mm | 4.5mm | 4 mm | 3.5mm | 3mm | 2.5mm | 2mm |
| Hook size | #2/#4 | #6/#8 | #8/#10 | #10/#12 | #12/#14/#16 | #16/#18 | #18/#20 |
