Could you explain sentences below explicitly, caring about the meaning of 'for'?
For a man of his wealth he's not exactly generous.
He plays very well for a beginner.
It was a difficult decision, especially for a child.
The winter has been unusually cold for Florida.
For every two people in favour of the law there are three against.
They are from Cambridge dictionary with the explanation that 'for' is sometimes used for comparing one thing with others of the same type.
Comparison can be seen in all your examples.
For a man of his wealth he's not exactly generous.--Though he has got more money than others, he isn't generous.
He plays very well for a beginner.--Though he is a beginner, he can play very well, better, than others.
It was a difficult decision, especially for a child.--The child made a decision, which would be a difficult task even for adults.
The winter has been unusually cold for Florida.--This winter has been unusually cold (compared to previous years).
It is not easy to define prepositions. The object of preposition for in these examples is the entity that is to be taken into account in respect to the predication made. In these examples, for has a hint of concession about it, as V.V.'s paraphrases attest. It might be paraphrased as "Although" or "Though".
For a man of his wealth, he can hardly be called generous.
Paraphrases:
Although he is a man of considerable wealth, he is not nearly as generous as he could be.
Considering the extent of his wealth, his generosity is quite meager.
The winter has been unusually cold for Florida.
Paraphrases:
Although it is Florida, the winter has been cold.
Considering Florida's usual warmth, this winter has been exceptionally cold.
