Mini micro nano ….pico?

Been looking into another project and “found” “new” ( 1 month old??) micro,
IMHO overpriced by comparison with well know market place, Arduino compatible hardware – USB dongle style.
What got me puzzled is rather typical marketing hype - both “Arduino compatible” and “micro”.
So what really makes hardware Arduino compatible and what is the next “size” - perhaps pico?

What makes it Arduino compatible is running it at 16 MHz so an Uno bootloader will work (or 8 MHz and a different board type, like 3.3V Promini).

There are several small versions of cards - the Promini, other surface mount variants (like mine, with offboard USB adapter needed and crystal and some Rs/Cs on the back side), and even Femtoduino, using the leadless Atmega chips and smaller pitch connector holes.
I see Picoduino on google, using Attiny85 chip. IDE can be added to, to support the chip.

Certainly pico doesn't follow micro, which is followed by nano, and... Oops, that has been used, so OOOOK, just use pico then.

If you are familiar with leonardo, micro is a miniaturized leonardo. All its pins etc are compatible with leo. You get HID keyboard and mouse support, separate hardware serial and usb virtual serial, very nice to have one on xbee and the other printing debug info. It also has many pwm and analog pins, separate spi pins etc. Don't mistake it for another flavor of UNO.

It is small and that is advantage.