It is recommended to start with stringing wires no larger than .013inch / .33mm. This will ensure a proper fit and in most cases allow for feeding of the wire tip back through the beads for a clean finish.
Elastic cords can be used with beads 4mm and larger, but it will not fit through most beads smaller than 4mm. It's recommended to use 0.5mm elastic cord.
These recommendations are intended to help guide you for a successful project. With time and experience you'll discover what works best for your designs.