Where do I find the answer of: What affects the length of a bridge, ie. cross sectional area, construction material, change in temperature or the length of the bridge?
The LENGTH of the bridge depends upon the body of water or valley that must be crossed. The other parameters (structural materials, length of span, structural design type, etc.) must be designed to satsify the length requirement and the width/load (number of lanes), temperature extremes, maximum wind etc.
There is a lot of use of computers to come up with a design that meets requirements at acceptable cost. Standard formulas used to compute stress in load-carrying members, to make sure maximum stresses are consistent with a suitable factor of safety.