This time of year there's always one bigger than the others - usually in the upward oxter (axil) of the last "big" leaf (ie. one that isn't smaller than the ones below).
You can get all the others before they get to 2cm, but this one will always make it to 4-6cm in the same time - never mind, it will root in water and make a good strong plant that will only be a bit shorter than its parent, and only a week or so later to crop.
Really useful for getting more from those "special" tomato plants you paid though the nose for.
Cheers.