Most codes describe usable, habitable bedroom space as having 7.5 feet of head space with a minimum room dimension of seven by ten square feet. Bedrooms can be invented in a variety of spaces but they might not have enough headroom needed to qualify as a true bedroom.
Short answer is - No.
Some older structures may have grandfathered aspects that although they currently do not meet building code, did meet codes at the time of construction.
Generally speaking a bedroom is required to be 8 x 8, have a window large enough for a fireman to crawl through, and a closet. However, in terms of value, calling it a den "that could be used as a guest room" might make sense.
Talk with a Realtor who knows the local customs.
You might want to contact Brokers in your area and get their opinion. If 2-3 say it is not a bedroom because of the size and lack of closet, I would consider that they would know.
I'm not sure if there is a specific minimum size though ... that 6x8 is really small. Your town will have codes that govern that - easy enough to check ...