In reply to ojp:
Are you leading on a single rope? if so you'll be needing to extend runners a fair bit. If you're using doubles I'd still err on the long side. After all it's much better to fall 3cm further but have your runner stay in!
If you're only buying five to start with then for trad I'd get something like 2x12cm, 2x15cm, 1x18cm.