I would step up all the way to a one gal. once you have four true leaves. I base this in part on the time of year; spring is coming and they will want to grow. Once they are established in their new pot start fertilizing - either use a timed release fertilizer (my preference) or use a regular (always well-balanced) one at half strength. Be sure not to push 'growth' too much, or you'll end up with a lanky plant.
