sizing wires and fuses

when we are designing the wiring harness shouldn't be worried about the voltage drop across the wire...if we have selected a wire and for it's length and the current it draws , has a large voltage drop (lets say 0,5+ volt) shouldn't be aiming for a larger wire ?? and what about the sizing of a fuse, what is the safety factor we apply to the peak current to choose the right fuse size..??

You can calculate a wires resistance from it's diameter and length, but most of this work has been done for us. Just google "wire size" and click on images. Literally hundreds of charts are already done showing voltage drop over length for a given wire, and recommended current for a given wire.

If you are getting a 0.5v drop, then go for a bigger wire yes.

A fuse could be sized by max current x 1.25 as a rough suggestion.