The shape and the size of the beads impact on the final look of the craft or jewelry piece that you are making.
For example, if you are doing an earrings project, you can opt to use crystal beads or seed beads or glass beads. If you want something to be your focal point, you can opt to use novelty beads such as butterfly shaped or cross shapes. Long tube shaped beads are good too as they lengthen the earrings but do not make the piece chunky.
If you are making a necklace project, maybe you might consider using special shapes such as star shape or small tubes or Celtic cross. If it is something simple and classic, you can strip a special shape bead on a pendant hook and string in a leather strap.
Other shapes such as large rectangular beads flower beads, square beads or circle hollow beads are good focus beads too. If you want something that is flat, you can use round shell pieces in your jewelry making.
However if you are doing embroidery, you might want to use a combination of seed beads or small crystal beads to add on to the glitter and sparkle.
For ethnic or tribal pieces, you might want to use odd shape beads such as leaf shape or big chunky pieces to create the look. Alternatively, you might just want to paint your own beads to the design you prefer.
Metal beads are good and are readily available in all shapes and sizes. They are heavier and thus you might wish to do a combination so that the jewelry piece is balance and not heavy on the neck or ear. Metal beads are good for creating the antique looking pieces. Wooden beads are usually bigger and might be more suitable for necklace or bracelet projects.
Beads Basic provides information on beading.