really small speakers less than 500

New or used?

A 6 inch woofer isn't really small. Lots of good little speakers with 4-5 inch woofers.

Cabinet size? i.e., no bigger than, e.g., 14 by 6 by 8? Smaller?

Since you define "small" as having up to a 6" mid-woofer, I'll assume you will consider small monitors. I recommend you look at the Krix Equinox. It's reasonably small (front dimension is about 7" x 11"), but is an outstanding little monitor. I have been using a pair in my office for about 6 months, and I think they are terrific! They sell new for $600, but I bought mine used on Audiogon for $300.

Maybe the B&W Leisure Monitors. Under $350, and they are versatile and stylish. Although limited in the bass, they can begin as a main speaker, be used as a rear surround later, and end up in a bedroom or porch even later. Has a built in stand and wall mounting system. Mine are being used as rear surrounds now, and have that classic B&W sound. Maybe not the "best", but one of the better brands.

Check out a pair of used Linn Tukans. They are about as good as it gets for their size.