One way to gather the information you need is to look at the franchise information on Gas Station or Travel Centers. While you may not want to purchase a franchise, their disclosures available online will broaden your understanding of this business. Additionally no one can give you a single firm answer to your question for each location will be unique AND the features/benefits of your station will be unique such as are you going to offer broader services like food or repairs or just offer gas?
So read up on business plans and articles like: (if you read through this first link you will find the details you wanted for this design)
http://www.bplans.com/convenience
storegas
stationbusiness
plan/executivesummary
fc.cfm
AND
http://www.buyerzone.com/professionalservices/franchises/tmm-gas-station-franchise.html
and
http://www.franchisegator.com/hoovers/Gas-Station-Franchises/
.... and there are many more.
Hope this give you a next step to your answer.
Alice, SCORE Pittsburgh
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