You can braise larger, thicker leeks, but I find smaller, thinner leeks tend to be more tender and so work best. If you have larger leeks, you might want to cut them into roughly 1 - 1 ½ inch (3-4cm) rounds instead of half-leek lengths and instead cook them on the cut end, and brown a couple minutes each side. Thicker leeks may need a slightly longer cooking time while covered - in all cases, check with a fork that they are tender through to the middle.