Just saying the TARDIS is bigger on the inside doesn’t come close to describing just how vast her interior really is. Not even the Doctor, in his centuries of travelling the universe in her, has ever managed to map her interior in its entirety.
In some respects it would be pointless to try, since she’s constantly changing, adding new rooms as required, occasionally discarding areas that no longer serve any purpose. She’s still evolving after all. She might be considered obsolete, but she’s a living, growing entity.
The Doctor and his companions still have a lot to discover about her.