I'm Brazilian and see all answers here as correct.
But, although the word 'disponha' has, indeed, many meanings, it's far more used as a reply to a 'Thanks' sentence and the meaning of it in this context is exactly this:
It is a contraction of 'disponha de minha ajuda sempre que precisar' (You can count on me whenever you need) as this answer points out.
The word 'disponha' on this sentence can be exchanged by 'Faça uso de minha ajuda sempre que precisar' (something like, 'Make use of my help whenever you need'), but we never say it in any long form, it is always (and only) disponha.