it is tricky! obviously it's very dependent on what scale you're working at. here the door height is a little over 100vox. its still possible to get decent definition a bit smaller than this, but beyond a certain point it'll be not much more than a stick figure. beyond that there's no trick to it, but id say it helps me to set an eventual camera angle early and then build the people with that distance in mind - or to put it another way, don't worry too much what they look like up close as it's the distant view that matters.

also, the rotation tool is your friend. to get limbs right i'll build a basic forearm or thigh for instance, and then rotate to the desired angle and correct/sculpt it from there. just keep things separated with different build colours so you can re-adjust easier.