Even with minimal slope, you should consider the quality of the soil. If it's mostly sand, it'll drain away before the swale catches it anyway. In packed clay soils, swales will catch every drop, but may easily overflow if not big enough. Ideally you'd have something in between (Loamy soil). Since you're obviously looking at this from a planting perspective (more than drainage), you're target soil should be a loose loam, rich in organic matter. If you achieve that quality of soil, the following may be helpful. I saw a formula for swaling, but I'm just basing this on a vague recollection.
For every 50 yards @ 10° of slope, you dig a 1' wide swale, ½' deep. Walling it up on the down slope to get you to about 8" of depth. For every additional 50 yards or 10° of slope, you add 25% to the trench width and depth. This was based on 30" of annual rainfall.
That's a good rule, but the land wont always support those relative depth dimensions, sometimes it needs to be shallow and wider, or deeper and narrower. The important thing is that you maintain the same ratio, so as to keep the same volume of capacity. I believe it assumes the rainfall average is spread out over different times in the year. I lived in Arizona for a while, so I know monsoons would wash that out in a minute, even though the annual rainfall is only around 25". Of course monsoons and hurricanes are the exception, but account for them with deeper swales if that may be an issue in your area. The "formula" obviously neglects soil penetration etc, but it'll give you a crude idea of what's needed.