I am making a human model, however, it's exceptionally large, and even putting all the bones' extents to max (and even scaling the bones to be taller), they are unable to fill the entire size of the head alone. Any way I could fix this, or am I forced to resize my model?

in object mode (the arrow at top on right hand side) select object by clicking on it. hold in control button, and press left mouse button, move mouse to resize model. i would just start with a new skeleton.
hope this helped, pirate

Man... okay... any way to scale the size of models by percentages? I've gone and made a male and female model, and articles of clothing, but I need them to retain their relative proportions to one another.

Right click on the model and from the drop down menu select Scale>>>Amount.
Enter something like 0.5 for xyz, then do the same for all the other objects. If it's still to large keep repeating the scaling.

Try building things to scale where you can...If you're working in feet and inches in 3dc, drop a cube onto the scene and scale it's y axis to 6ft....then scale the figure down to this height.
Then if you want to build a scene for the figures you can make things like doorways 7ft tall...it's much easier this way than having everything at different scales.

Bazza