Periods as a whole are typically 8-bars or 16-bars1
Think of how you would sing each section (basic idea vs. contrasting idea vs. resolution) - each should probably be in a single breath
The "contrast" in the contrasting idea most often comes from melody and not harmony
NOTE: this is probably not the case for classical music!
The repetition of the basic idea can be different from the original statement of the basic idea, but it needs to be more familiar to the listener than the contrasting idea
For modern music (film + video games), the antecedent needs to simply have a weaker cadence than the consequent phrase2