My opinion would be to look at Better Bottles instead of glass. My first carboy was glass. I don't use it anymore. They are heavy, slippery, and can be dangerous if dropped. There are some threads on here talking about this. Better Bottles are cheap, durable, and easy to clean. They're made from food-grade plastic.
As far as size, I ferment in a 6 gallon carboy, and If I secondary I use a 5-gallon carboy. That would be for a 5-gallon batch. You need the extra head space during initial fermentation to prevent a blowoff, so a 6.5-gallon carboy is used. Of course you can always use a bucket as well.
Never underestimate the potential of someone who refuses to act their age.