It’s just what I look for: a bit stretchy, just above the knee, breathable. Just above the knee is crucial, because it allows full leg movement without getting tangled in my rear brake pads (say) but doesn’t ride up enough to give the entire bike route a view of my knickers. Yay!
My other secret weapon in the cycling in dresses department is Jockeys Skimmies Slipshorts. Ignore the naff name, enjoy the comfortable, non-chafing underwear. I particularly like these as they aren’t meant as shapewear, so there is no Olympics-worthy, sweat-inducing 20 minutes getting them on. They come in many colours and the bottom edge isn’t lacey or anything, so if you do accidentally flash a passing cyclist, you just look super prepared in bike shorts. They are also quite cool to wear.