Traditionally, Leeks get planted out when they have reached "pencil size", but I have planted some of mine while they are still smaller than that (they seem to be taking an age to bulk-up). The reason for this is that they are going in between rows of Parsnips, and I'm worried that the Parsnip foliage will shade them too much before they have a chance to get established. Look, these are quite decent seedlings, even if they're not as fat as a pencil. They certainly have nice strong roots.
Fortunately, I remembered that I have a really efficient dibber, which is perfect for planting Leeks. Last year I forgot that I had this one, and used a different approach. The sharp metal point penetrates the soil very easily, and the long wooden shaft makes a nice deep hole for the seedlings to go into. Leeks are typically planted pretty deeply in order to produce long white shanks, which are more tender than the grey/green leaves.
Here's a photo of a Leek seedling standing in its new home. You just drop the seedling into the hole and then water it in. You don't even need to backfill with soil, because the act of watering will wash soil into the hole and cover the roots.
So now I have a row of 14 Leeks between two rows of Parsnips. I'll do another row of Leeks whenever I have some more that look big enough - hopefully within the next 10 days or so.
The Leeks I planted here are "Toledo" ones, and the ones waiting in the wings are "Apollo" and "Winter Giant".