I kept answering, "None fit." There are definitely patterns in the way I ship, just not hair patterns. The "relative size" question was actually the only one I could have answered with some consistency, if only you had included, "big or small, but more or less equal," but even then...le'see...

Buffy/Spike: blond/blond, short/short, good/redeemed, young & stupid/old & stupid, notably antagonistic at some point in relationship
Aragorn/Arwen: dark/dark, 6'+/6'+, good/good, young & wise/old & wise
Thomas Mendip/Jennet Jourdemayne: ?/?, ?/?, good/good, young & smart/young & smart
Neville/Draco: brown/blond, [tall in my fanon]/tall, good/redeemable, young/young
Harry/Draco: dark/blond, [tall in my fanon, because they must be nose to nose, as adults]/tall, good/redeemable, young & stupid/young & stupid, notably antagonistic at some point in relationship
Ellen Fanshaw/Geoffrey Tennant: reddish brown/dark, tall/taller, middle-aged & fucked up/middle-aged & bonkers, notably antagonistic at some point in relationship
Roland Mitchell/Maud Bailey: dark/light, short/tall, youngish & smart/youngish & smart
Elizabeth/Darcy: [dark honey-blonde, in my head]/dark, ?/tall, young & smart/young & smart, notably antagonistic at some point in relationship

The single constant in my OTPing seems to be that all these people have something vital to teach their other halves. Lessons may involve a lot of squawking and snarking. Certain equalities are important to me -- not necessarily age, but experience, or ability to process experience.

You have noticed that H/D = B/S, ja? *cackle*