Economically, when it can no longer support itself and it's programs. Socially, when it begins to regulate consensual adult behavior and endeavors to engage in programs of social engineering against the wishes of the people.
Well reasoned but the economic arm of your contention isn't there any more. The fed. gov. is monetarily sovereign. It does not use tax dollars to fund its escapades. The wealth of the nation is tied to its full faith and credit and not the value of gold. The only true limitation to spending is inflationary concerns and we've been dealing more with deflation. (the debt ceiling argument is an anachronism from the gold std days and has no place in our gov...it's merely polkitical theater).
I think gov. can over reach, it can be bad governance or good. I think it's a values call in the end...just like everything else. Big gov. does not mean bad government any more than small government means good government.