There are a number of factors to take into consideration, such as current sq ft living space of your home, the typical homes for sale in your neighborhood (how many have garage conversions), parking situation if you take away garage space, etc.
Whatever you decide, keep in mind that garage conversions require permits. Converting a garage to living space without the required permits could lead to problems later on when you sell your home. Lack of required permits will lead appraisers to discount the new sq ft living space. Buyers will also run into problems obtaining insurance. Even if you do pull all required permits for your conversion, many buyers actually want a garage, so they will skip your listing when searching for homes.
Would suggest weighing the pros and cons before making a final decision on garage conversions. Bigger is not always better.