I concur that most powered flying needs about 2000' side-to-side. While I'm sure 500' is adequate depth, I think a bit more, say 700-800', would be noticeably more comfortable in many instances since it should provide for cross-wind 'slop'. The comment about sun position and prevailing winds is excellent, and should be a prime consideration, just as distance from an airport or approach.
As someone who lives on (fairly large) acreage, and who spent considerable time looking at acreages before purchasing one, I expect that finding something that fits those long, narrow flying dimensions efficiently (so as to minimize cost) could be quite a challenge (at least it would be in my neck of the woods). That's not to say that those shapes are not out there, but it could require a lot of hunting, and when you find it, the other parameters (i.e., contour, orientation, favorable winds, distance from airport, etc.) are increasingly unlikely to also be present. So you may likely have to purchase something considerably larger than your needs in order to get a useable 2000' length.
I assume of course that you have other reasons for buying acreage, so you may already have given the following some thought. My experience, however, watching people move out to rural areas (like where I live), and then selling again within a couple of years, suggests that at least some folks don't give much thought to the differences encountered living in a rural area. If you've never lived on acreage before, don't expect it to be like living on a city lot. Your services will likely be quite different and considerably more costly. Electricity will be the same (TV might be too), but you can expect to shell out for septic and water, trash pick-up, and maybe even Internet if you have to rely on satellite. Natural gas for heating (if necessary) may not be available. Your transportation costs (including obtaining groceries) will likely be higher. Services like snow removal (depending on your latitude of course) could also be an issue, and even your choice of vehicle could be affected if you choose a location on a less-than-ideally-maintained gravel road and have to be concerned with spring mud and winter snow. If you choose a property outside city limits, which would generally be the case for most acreages, you might also expect to pay more for technicians and craftsmen (electricians, plumbers, roofers, etc.) that might be necessary for repairs you can't manage yourself. In my situation, I live only 15 min., by good paved highway and rural road, from city limits (~90,000 popn.), and yet I'm generally charged between $100-$200 for "travel" just to get these specialists to the ranch. And then there's the property taxes; the more acres, the higher the taxes of course, and while the tax rate is generally lower than inside city limits, you will likely be paying considerably more in taxes due to the much larger property (unless you can benefit from something like obtaining farm status, which, where I am at least, comes with a reduced tax rate).
This is just meant to be some food for thought. A rural lifestyle is wonderful, but it is a "lifestyle", and those folks who attempt to live a "city lifestyle out in the country" are commonly disappointed. That said, if you think it might be a good match for you, then I highly recommend it.
