A couple of weeks ago, I quoted the four times in the Last Supper Discourse Jesus told His disciples that anything they asked for would be given.
There's a fifth time in St. John's Gospel that the "anything you ask for" construct is used, and it may shed some light on what Jesus was getting at:
Martha said to Jesus, "Lord, if you had been here, my brother would not have died. But even now I know that whatever you ask of God, God will give you."
And of course, what Jesus asked is that Lazarus would come forth from his tomb.
So that's the sort of thing that will be given if asked for.
St. John takes great care to recount the deliberation with which Jesus acts -- beginning with His decision to not act, to remain where He was for two days. Contrast this with the miracle at Cana, which Jesus seems to perform rather casually. Yet in both cases what is asked for is given.
I'm not sure which is the more astonishing sign. I mean, raising someone from the dead is just the sort of thing you'd expect God to do. But a hundred and fifty gallons of the best wine? For a party that had already gone through all the wine they had? Isn't that a bit... frivolous? Yes, it's different in important ways from giving a bicycle or a pony, but it's certainly looking in that direction.
I'll suggest, then, that Jesus' promise of whatever is asked for does not depend on the magnitude or the necessity of the thing asked for.