Determine how you will use the stage. You will have to let the stage rental company know how many people you will be seating, or if you will need a runway. A band or play may require more stage area then you expect. By giving your uses to the rental firm, you can let them suggest the right size for your event.
Find out how much assistance you will need to erect the stage. Most stage rental companies will install and take down their equipment. It will cost more, but may be worth the price if you don't have any experienced help.
Consider your guests who will be using the stage. You should plan to provide a ramp if you have any guests in wheelchairs. Other handicap options you might consider include handrails and guardrails.
Think about the overall presence of the stage and what kind of décor you are using for the event. You should plan to rent the skirting and backdrops from the same place where you rent the stage in order to have a professional look and make sure that everything fits correctly. Decide if you need to rent any furniture or A/V equipment if you are using the stage for a theatrical, public speaking or musical event.