If you want to put 1 person in a 3 man tent, stop pretending you care about weight.
Go to a reputable backpacking gear shop midweek during late morning. They'll be slow for customers and have enough time to help you explore lots of different options. There's no substitute for actually laying down inside a sleeping bag or tent.
If you're taller than six foot, you'll probably want a size "long" sleeping bag.