The minimum width of a strip depends on a number of factors including pilot skill, but about 50 feet is probably as narrow as you would want under favorable conditions. If you have trees on both sides of a 50' wide strip, you only have 7 or 8 feet clear off either wingtip between you and disaster. If you commonly have a cross wind you need to allow a little extra room for that. If you have trees or buildings on the upwind side of a runway that often gets a corsswind you need to allow for the turbulence that those obstructions will likely generate. If the area on either side of the runway is relatively flat and clear you can more easily consider a narrow strip. If night operations are a consideration you should start thinking about a minimum width closer to 100' that 50', again depending on surrounding conditions.
High wing airplanes can use narrower strips than low wings if the obstructions along the sides are not too tall. There is actually a strip on the FAA charts that is only 8' wide in Alaska, but it is pretty much a Super Cub only strip with low brush along both sides, not exactly the kind of place you would want to go with your Mooney.
The spot I'm thinking of is owned by family, outside the city limits, and aligned with the prevailing winds, and maybe 600 ft. There's a drop-off (maybe 4 ft) on one side and a barbed wire fence on the other and I need to measure the potential width. It might work. Thanks for the responses.
600 feet is pretty short. It's doable for a wide range of reasonably STOL designs, but it's going to take some training to become comfortable with it.
Another factor for a strip this long is obstacles on the approach/departure. If you can pass over a three-foot fence and hit "the numbers" twenty feet in, it'll help a lot; if you have to clear a thirty foot tree (or stay high to keep neighbors happy), that can use a good 100+ ft of your strip. Same thing on departure.
I have seen a J3 land at my RC club, 600ft, but being an RC field, no obstacles. I think the Van's company strip is about 1200ft. Crop dusters land with little more than the wheel track. Pick your plane well.
Narrow strips make crosswinds very tricky. I used to carve temporary strips out of crops like wheat or canola for advanced off-field student training, and made them 50' wide. That's a bit more than the wingspan, but when there's tall crop like canola you sure don't want to snag anything in it. It's tangly stuff and will pull the airplane into it instantly.
It wasn't grass but I landed a CL-60 on a strip that was part of a private ranch in Texas years ago. It wasn't more than 10 - 15 feet wider than the main gear and the wingtip hung over the fence that ran the length of one side. The pavement had the appearance of an old asphalt driveway.
I'd say the width depends mostly on pilot skill, surface conditions (wet, dry) crosswind component and the distance of any lateral objects just off the runway.
My rule is 150% of wingspan for wingspan in calm winds or winds under 25% of the maximum demonstrated crosswind speed; 175% for VFR operations with winds not meeting those criteria but <75% of max demonstrated crosswind speed; 175-200% for IMC operations or anything >75% max crosswind speed (the latter usually means we're going somewhere else or using another runway).
Keep in mind that I don't fly from grass strips so there are based on a solid surface runway.
Length: 2.5-3 times the ground roll @ MTOW, no wind and on tarmac will do for almost any grass strip.
Strip width: I've flown a number of times from 4' wide strips, no big deal, as long as the fields on the sides aren't too rough and you have a single wheel. For a Cessna or so, I'd say 25 ft will do.
Clearance though is far more important. 50 ft tall trees will add about 1200 ft of required runway length. Sideways I'd say you want 200 ft at least, clear of any obstacles more than 1 ft high. In windy or mountainous areas, you might need a lot more, to account for wind shear and rotor.
Most of the bush strips I know around here are no wider than the width of a Cessna's undercarrage plus a small margin. Maybe between 4m and 6m all up. But that said, there's not defined 'edge' with many of them, more like things just get bumpier the further you stray off the tracks. But others are cut straight into the brush, with no margin for error. Typically they're not more than 300m-350m long.
I'm not suggesting this you'd want that for your home field, but its an interesting example of what some pilots deal with on a daily basis. And it shows what can be acomplished if you fly carefully and accurately.
My father (along with many others) flew and still fly out of strips of this size for much of their working life, without incident.
It is neither a function of horsepower or even wingspan. It is largely based on a combination of the approach speeds of the aircraft and the pilot's skill level in that particular aircraft. Another factor is the forward visibility in the final stages of approach and landing. How gusty the winds are in your area can be a consideration as well. Think about the paved runways you're now flying off of. How much of its width do you typically use when you're a bit tired and the wind is a bit gusty. You'll want at least than much for your own grass strip if you don't want the strip to limit your flying.