I wonder If there are tubes smaller than T5F5.

4W T5 is the smallest standard fluorescent lamp that available commercially, but there were fluorescent nightlights with even smaller tubes.
Here are the smallest prototype hot cathode fluorescent lamps made: https://www.lighting-gallery.net/gallery/displayimage.php?pos=-69626
And here are the smallest prototype cold cathode fluorescent lamps made: https://www.lighting-gallery.net/gallery/displayimage.php?pos=-69625

The smallest I've seen in stores was a 1 watt CFL sold as replacements for a solar powered floodlight. I've never actually seen the fixtures they were designed for though, as by the time fluorescent solar lights have already been killed off by LED solar lights.

There are lower wattage fluorescents, but mainly the CCFL type, due to the efficacy reason (keeping cathodes hot requires power which is not scaling down that much).

I have seen 1W hot cathode wired in a standard hot start electronic ballast.
Then the cold start nightlights are quite common too, but these have the filaments shorted and use auxiliary probe to start.
But the lowest standardized power rating I know is is indeed the F4T5...

I have seen some of those small hot cathode in use as instrument lighting at Air Products in Chandler, AZ.  Probably worked better for them since the lights are on 24 hours a day.

I've seen small fluorescents in some night lights.