In any real-world application of socialism I've seen, it is indeed the government deciding how much one is given. I can't think of any implementation where the government asks the citizen how large of a slice of the pie they'd like.
I'm happy to be proved wrong, and maybe someday there'll be a successful communist state, but until then I look to history.
The person defines what they need, not some third party who has obvious conflict of interest in down-estimating the needs.