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I've started growing a tomato plant a month ago and it is still pretty small. Around 7 cm high.
I grow it with a small fluorescent light ( T8 model ) in my basement.
Why is it growing so slow?
Regular T8 is probably not giving enough light, the High Output version is usually used for tomatoes, and that version is referred to as HO T8 Growlight fluorescent tube rather than just T8 tube. If ordinary fluorescents are used, these are more usually T12, and are much better at growing leafy crops such as lettuce than fruiting ones like tomatoes.
Tomatoes need a minimum of 8 hours darkness per day, between 8-10 hours is optimum, so the lack of that, coupled with the use of standard T8 lights, might be the reason. Note also that plants grown where air is wafting around them (not blasting) grow stronger, with thicker stems, than plants in an area where air movement is virtually nil most of the time, due to a process named thigmomorphogenesis (just in case you want to look it up).
The optimum distance between the plant and the light should be between 11 - 51 cm, but better results are achieved with the lower measurement (4 inches, or 11 cm), so moving the light closer would be a good idea.
On the pot size, once it starts growing properly, it should be in a pot that's an 8/9 inch tomato pot, so around 23cm, for good cropping, with the usual high potash feeding regime.
