Elaine, at the risk of a serious injury from our resident Leek King - DD, I'd advise that you'll be fine when they're biro refill size!
I have actually taken his previous advice this year and transplanted them into larger pots this year, because I want to plant them out differently, but in the past, as long as they're eventually large enough to be a couple of inches above the soil-level when planted out, and watered in properly, you should be just fine!
Just a thought though, are they earlies, mids or winter ones?