Transformer maximum voltage rating specifications
For a flyback design, your principal design considerations will be primary inductance and the turns ratios from primary to each secondary. You need to design according to the specs for your flyback controller IC. That will give you the information you need to specify transformer. Flyback as well as other switching topologies, allows varying input voltage such as 15-40, but with stable output voltage voltage.

To answer your question more directly - output windings conductors size is chosen according to maximum current rating - drawing more current than the transformer is designed for will increase internal temp of transformer.  Output Voltage is more related to turns ratio, but is not just a simple linear ratio for flyback design or other switching topolgies. 

With respect to output voltage of 75 to 100V, assuming same turns ratio, your input voltage range would be similar of 75 to 100 or so. This would work fine if input and output voltages were increased - but again, you need to choose based on primary inductance required, and that will vary by overall design. You also need to be careful not to exceed power dissipation in transformer or you will have serious thermal issues. 

Not sure why you would consider flyback with turns ratio of 1:1 however.