Morph - whatever YOU like. Not much fun to clean up poop after an animal you're not crazy about for the next 20-30 years!
Age/Size - baby to juvenile would be a good start. But decide on what size you're looking for ahead of time, and get your enclosure set up, and running with belly heat and thermostat a few days BEFORE you bring home your new addition, so you can be sure everything is perfect for them.