I'm not sure how small we could be, we have way too many global business interests. Also we shouldn't turn a blind eye towards mass genocide and other wrong doings done by one group to a another group.
Sigh. Only way to end genocide in the Balkans would be to sterilize the entire region with radiological and chemical weapons. Seriously, often that can be the only way to end genocide. By wiping out one or both sides.
After having done that "nation building", I'm very skeptical of it. Very expensive, very little gain, etc. It's a bit different if you're doing so with a country that has been fairly well organized prior to being wrecked. Third world countries? Not so great.
So what should our engagement window be? Simple, least amount of stuff we can get away with. Fair treatment to everyone then subject to merit. Mirror other countries' trade practices. If they tariff our stuff at say, 60%, we should do the same to their goods. 0%? Sure. Why not. A respectable Navy to keep our shipping lanes open in case someone gets miffed that we want even and fair trade. Enough Army to defeat whoever else is largest. USAF to run satellites and keep air superiority. Drastically cut foreign aid, which tends to be corporate welfare scams anyways. Slap the ever lovin' hell out of the CIA, to remind them that their job is intel gathering and not "rigging nations". We need intel agencies, but they should not be making policy. Don't try to rebuild third world countries. Just don't. It's not worth it. If you REALLY want to help them and THEY ask, send advisers with armed guards and that's it. Have counterintelligence personnel closely watching the State Department, intel folks, etc.
If a couple foreign countries are angry at each other and it doesn't influence critical resources, respond promptly with "That sucks." Mutual defense treaties only with folks really in our interest. Decent folks, good trade relations, relatively civilized and not stirring the pot? Taiwan or the Ukraine comes to mind. Sure. Why not?
I'm not saying being isolationist. I'm saying "Don't poke your nose where it doesn't belong, isn't appreciated, won't fix anything and will cost literal metric tons of cash." If it makes sense, it makes sense so do it. If not, stay out of it.
Thank the Gods we DID turn a blind eye to Darfur, dude. And a hundred other genocides. Folks do not like world policemen. Nor should they. If we feel the NEED to get involved, send the weaker side a couple C-130s with AKs, ammo, some food and medical supplies. Then call it a day. If you really want to be small government type, make it optional line item on tax returns. "Do you want to sponsor American international intervention in other people's problems? Yes? You can donate 1% of your tax return to sending an AK to ... where ever."