How man y watt FPL light tube should I use?

Hi

I am non from electrical background.

At my place there is box say don't know what to call this say light box .
It has choke fixed of 40 watt as we found in old age tube light plates.

There is place to attach FPL pattern bulb/tube.

I am not sure how many watt FPL/Tube should I use.

Is there any problem I should use 11 watt FPL ?

Whats the logging behind choosing right watt of bulb?

Thanks