The long answer
A lot of this is determined by your design configuration. To get close to knowing the limits of a particular design or structure you first need data. You will need to know many variables including shear strength, load data and the operational environment your materials will be required to operate in. To determine accurately what your design is capable of you will need to know beforehand the safe limits of your materials and then design and build within these paramaters. This can get complex. For example, steel would give one value, soft woods such as pine or fir would give another, more dense hardwoods such as oak or maple would give another. Each of which could use a different design entirely and produce very similar results. This is why most people tend to err on the side of safety and copy an existing design or simply overbuild it. This becomes more critical when the values go up. If you really want to learn the engineering aspect of structural design I would recommend taking a look at bridge building.
The short answer
Copy an existing design.
