Hey Dude, its not a stupid question at all. If its in good shape, it should be fine. I use a 5 gallon glass carboy for a secondary/bright tank. If its exactly 5 gallons, it might be a little small for a primary fermentation, if you do the standard 5 gallon batches. The krausen would probably push out the carboy during the fermentation. But you could do 4 gallon batch in it with no problem. It might be worth measuring exactly how big the carboy is.
Congrats on the free carboy!