Despite diverse approaches at trying to tax its citizens, the federal government has collected revenues at an average around 17-19% of GDP most years over the past several decades. So right off the bat, I think government spending at 19% GDP should be the target on the high side. (Hey, I'd prefer it to be under 10% considering it averaged that through most of this country's existence, from the very beginning up to becoming an economic world superpower). In recent years spending has been close to 25%, and I don't buy there is a tax scheme out there that will extract the necessary revenues needed to sustain it.
I'd start with federal subsidy programs. We have over 2,200 right now...we had 1,600 only as far back as 2005...just 1,000 back in 1985. As much as I'd like to live out my libertarian fantasies and dramatically reduce the size/scope of gov't (including radical entitlement reform), we don't have to go that far to make a difference. Food subsidies, farm subsidies, subsidies to foreign governments, subsidies for energy, housing, public broadcasting. I'd like to wipe them out, but even if we could roll back spending to a decade or so ago, it would be a massive improvement over what we have now.
Considering the Department of Education has exploded with no discernible improvement in education, I'd make dramatic cuts there. I think the amount of money we spend on military/defense is beyond excessive, so I'd cut there. I'd end the war on drugs and allow states to make their own decisions regarding how to deal with them so we stop wasting billions of dollars there.
The problem is, any proposed cut is called "draconian." Heck, I started a thread here about the federal government spending taxpayer dollars to plant trees in the front yards of millionaires and had liberals defending that policy! Forget cutting government...we live in an age where if you propose to increase government spending less than it's projected to increase, that's somehow passed off as a spending cut. If I decide I'm going to spend $20 more dollars than I usually do on groceries this week, then I get to the store and only spend $15 more than I usually do, in Washingtonspeak, I just decreased spending $5, rather than increase it $15.
So do I have much hope that we're going to roll back spending at all? No I do not.