Tomatoes being planted out in the garden with three or four true leaves developed.
Generally, with all types of seedlings, they are best transplanted into the garden when they have a pair of 'true' leaves - not the first seed leaves. Most vegetable seedlings are big enough at that stage.
A good healthy Swiss Chard seedling - ready for planting out in the gardenafter being grown in a peat pot
However, with vegetable seedling - which are normally transplanted into the open ground - not under cover seed boxes - so they need to be a little bit more advanced than greenhouse transplanted seedlings. Two full pairs of true leaves is a good guide, but in the case of lettuce, then three or four true leaves should be allowed to form. The actual size of the seedling to be planted out, will be determined by the variety. For instance, cabbage plants will be much larger than lettuce plants when they have two pairs of true leaves.
The main exception for seedling size is with root crops. These should always be planted in the garden as small as possible - not allowing them to get too big, and certainly before the root system starts to swell. They will need extra care and attention - particularly with regards to drying out before established. If they are grown individually in small peat pots, then the problem is not so acute.