No, that is not the definition. Stealing is taking something without the legal right. Taxation is legal. Therefore it is not stealing. It is not theft. This is true regardless of whether the tax money was well spent by the government, was given away to someone else, or was just thrown away.
That question is a deflection, because one does not need to specify how much should be spent to maintain that doing so is not intrinsically immoral. Note that in specific instances a person might reasonable conclude that certain wealth redistribution is immoral. But you are not even attempting to do that. You seem to be trying to claim the much broader claim that any redistribution program is immoral, which of course is false.
These questions you pose are not unique to redistribution programs. They could be asked of any number of government programs. Taking public schools again, who gets to decide how much tax to collect for schools, and why, and on what basis? How much school taxes is moral and how much is immoral? These are difficult questions to answer for school taxes, and people argue about them all the time. But the fact that they are difficult questions does not prove that school taxes are immoral. Similarly, the fact that the same questions applied to a redistribution program are also difficult to answer does not, by itself, prove that all redistribution programs are immoral.
If you accept that conclusion, then I suggest you move on to the more relevant question of proving that the wealth redistribution implicit in subsidized health care, like a single payer system, is immoral. Make it easy on yourself and scale back your claim to one you might actually have a chance of proving. Since this thread is only about single payer health care, that is the only redistribution program you need to address.