First we had to decide what we wanted the boat for...sitting at the dock in condo like comfort, or cruising? Once my wife retires, we'll live aboard but be active long distance cruisers so we needed some of both. Do you want good usable outside deck space or a huge living room style salon? Remember, every large space requires minimizing another space. We had started looking in the 36 foot range but decided that no configuration of that length gave us enough living/boating space so we decided that we needed 40-45 feet. Since we decided on cruising, we now had to decide on power...We were looking for a single engine for economy, but they are few and far between so decided on relatively low power reliable, economical twins (perkins rather than Detroit diesel, Cat etc.) Next we had to decide on the galley size/position. That is galley up or down, and galley size. We'll be long distance so decided on a medium galley with full size equipment..3 burner stove, full size fridge. We elected a full size in a galley down configuration as that increases the salon size. Next was sleeping arrangements, what works for you? We wanted a centerline aft cabin for the master. Due to probably anchoring out rather than paying mooring fees every night, we decided a generator, and decent fresh water and holding tank capacities were important. Once those decisions were made...we set criterias in "Yachtworld and Denisons" personal boat shoppers and started looking at boats....This eventually created a short-list without spending $thousands traveling to boats that probably wouldn't fit our needs.
Hope this helps to at least get you started.
K & J