Best Character Head Size?
That’s an opinion is it not? 
If you really want some “data”, bring up photos of fashion models you find attractive and measure the ratio of head to body.
A rule of thumb I use is to increase the head size for shorter characters, decrease it a bit for taller ones. The reason is that the height slider is really a whole figure scaler, but human head sizes tend to remain fairly constant through the whole normal range of body sizes. I make the head narrower for slender characters, wider for stout ones.
To get your alts matching your aesthetics, you might want to make the alt, screenshot all the sliders (and face choice plus skin/hair/eye color choices too), go into game. Run around in circles a little, zoom in and out, decide if there’s anything you want to change. Note that down, then delete and remake using the screenshots as reference for sliders then tweak the sliders, screenshot the changes, and go in game again. Rinse and repeat until you are happy.
Instead of pictures of fashion models, I would look at an art primer. There will be advice/recommendations on relative head size and proportion there. I think it would be more relevant than static pictures, since you are actually composing figures yourself. As always, you are the best judge of what you like. Though your tastes may change/evolve over time.
Also, I really like what Donari wrote. It sounds like a good procedure for dialing in and finalizing your look, regardless of the method you choose to select it.
Bobblehead … drops mic
RIP City of Heroes
FWIW, I done a bit of drawing in my life. I use the method outlined in Donari’s second paragraph. Tedious, but worth it, given you may be looking at the toon for years. You can stand facing a wall, and use the wall to push your camera in for a closer view to examine details, but what really matters is how she looks with your normal camera position. Due to changes in LoD, these are not the same thing.
The actual daylight in the game is much harsher than that in creation (although the light in creation seems to change randomly), so beware the lighter skin tones and lighter blonde hairs. The next to last spot on the top row of skin tones still looks Caucasian in the game. More than 4 steps to the left starts to burn out badly in daylight.
As to head size, all the way to the right is still too small. But these things are relative, and you might feel you have a “big head” when looking at those around you. Donari is right in that the head scales with the height, so far to the right is the right choice for any height, but you might want to go a little smaller with a wider head, as that affects our perception more than head height does (head height is all the art books care about, but they’re just parroting classic painters). And, the “proper” width is somewhere far into the left half of the scale, depending on which face you choose.
BTW, you are correct that ‘B’ is the hottest human figure. The breasts protrude forward more than any of the others, and she does look slimmer in the hips with armor on. 
(edited by Finntaine.2067)