I'm not really sure I know what you're talking about, which probably means you don't really know what you're talking about either.

If you don't understand UV mapping, go through tutorials, there are countless ones available.

well, there are a few ways to go about uv'ing objects.
1.)unwrap your object and then use this layout for painting your texture
2.)make your texture and then unwrap your model to fit it (my favorite).

however the question you ask is not a simple one and may require you to lookup more information on your own. however the basics go:
-when painting your uv texture keep in mind your goal is to fit as much detail into a single map as possible, and to make it run as fast as possible. thinking this way we run into topics such as pixel density and how it relates to fill rate (search google). common methods for reducing fill rate in areas that are not needed as much and to increase detail in needed areas is to make certain areas takeup a larger portion of your texture. for example, you may make your characters shoe bottom texture very tiny in comparison to his face, even though they're near the same length, this is because you'll be seeing his face more than the bottom of his feet. this is why the guy's face is larger in that texture you supplied.
next we also have the common technique of mirroring, if the left side of your character is the exact same as the right, why not just completly cutout the left side's texture? so now your texture just has the texture for the right side of your character, then you can mirror the uv coords when unwraping over the right sides texture. to put it more simply imagine just modeling the right side of your character (some character modeling techniques do this), then mirror that side and what do you have? a full human! you're basically doing the same thing but instead with the texture instead of the model.
that should explain the basics, if you ahve any other questions just ask.

Jarrod's got it pretty much right, the thing I'd caution is:
" well, there are a few ways to go about uv'ing objects.
1.)unwrap your object and then use this layout for painting your texture
2.)make your texture and then unwrap your model to fit it (my favorite)."

The second way ONLY applies to environments. Characters, except for extremely simple ones (under 800 polygons maybe), must be unwrapped first and then the texture painted on that UV layout.

As far as the UV layout goes, it has nothing to do with character design or whatnot. The pipeline goes, I model a character, UV it, then arrange the UV's as required so I get a nice layout, then I paint the texture from that UV layout. The increased 'head size' only comes at the UV arrangement stage, its a painless process.

The art and science behind UV layouts is an indepth one so I won't go into it here but the last two posts laid out the gist of it.

To clarify, just so it's easier to find tutorials, the term you seem to be looking for is "UV unwrapping".

Here's a (random google image search) example: http://www.golemworx.com/pub/QuickUnwrap.jpg

You can see the geometry of the model has been "unwrapped" so it is flattened onto a plane. This results in a wireframe guide that you can draw over. The process of UV unwrapping is to create that guide. In many cases, a good UV map can be harder to make than the model or the texture itself.

There are various techniques for unwrapping that depend largely on the modelling software you use. If supported, pelt mapping is a more recent trick that works very effectively and very quickly. Often times the major 3D programs don't have good unwrapping support, but people will write plugins for them to make up for it. They aren't too hard to find with a little searching.

Quote:Original post by Cyntalan
see, that's the part I understand.

Now, painting that very flat image so that it looks right when wrapped. That's the thing I don't see covered, because when you go to paint that, if you attempt to paint in proportion, the texture will be bent along the polys that will cause a disproportion that was not there in the flat texture before. Working backwards, taking a texture off of a model already complete, the flat texture looks disproportioned (note my previous example, in how the face looks misshapen flat, but fine when placed on the model). This technique of expecting disproportion on your flat texture and painting it properly is what I don't fully understand.

perhaps you should go through a few uv unwrapping tutorials for your modleing program to get a better idea. it'll all make more sense once you try it indepthly.

Ah, I think I see what you mean. How do you accomodate for the texture stretching and disproportions when texture painting? Well, its something you just do as you texture, since you always work on your texture with your model constantly updating to check your work. You also use mesh topology to guide your textures (represented in the UV snapshot), so you know where the nose is, you know where the ear is, etc. You can also use a built-in 3dpaint tool to mark landmarks like this and do rough texture layout (as opposed to using a 3dpaint tool to make your actual texture). Is that what you mean? Regardless, I think the best thing for you would be to go over a complete character tutorial- model, UV, texture, in that order, and it will give you a better idea maybe.