Hi stuffradio,

Start hardening your plants off. (See item #9 in the FAQ on How do I start tomatoes from seed?) After a week or so of getting them adapted to being outdoors, you can go ahead and plant them in their final homes. (You might want to hold a few back as reserves, just in case something happens to the first ones.)

The article I've linked below explains the changes your tomatoes go through when you harden them off.

I am sure there are people who will disagree with me on putting them out as small as they are, but there have been studies that indicate plants that are put out at smaller sizes tend to adapt easily and catch up with plants that are put out at a later time.

Betsy

Here is a link that might be useful: Hardening Off and Physiological Changes

I agree with Betsy's post, I begin setting out seedlings as early as I can (without risking losing them) to begin getting them used to being outdoors. Depending on when I started my seed for the different varieties, they could be 3 inches tall or 6 inches tall - they begin to go outside whenever the weather is favorable.

Rather than growing them longer inside and giving them only a week or so of hardening. I like to transition them outdoors as soon as possible, during the day at first - taking them in at night if it gets too cold. I usually end-up doing about two weeks of hardening. The first few days they are in a shadier spot, with some protection from wind, so they can get used to the difference in humidity and the wind. After that I gradually start increasing sun and wind exposure until they are receiving as much sun and wind as they will be getting in the garden.

IMO, if the weather is favorable enough outside for seedlings, I'd rather have them out there than under lights - saves electricity to boot, hehe.