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The answer is:

since UAVs are supposed to fly gracefully,

if you pick a platform that is manageable in manual mode (not overly sluggish),

you can fly a machine of arbitrary size.

Also the stall recovery using rudder is intuitive.

IMO positive stability is not strictly required,

but for real-world applications aircraft stability and autopilot action should better be working together.

MultiplexFox (0.26kg, no payload, 20-45min enduirance, 45-55km/h)

Research plane - classified as Barely Flying Machine. -40deg glide slope when unpowered.

Autopilot is the supporting structure... (maybe World's first Intergral Autopilot Design ;-) )

EasyUAV (1.2kg AUW, 0.65kg empty weight, 20min-1h10 endurance, 35-45km/h)

Pteryx UAV (3.2-5kg AUW, 2.6kg empty weight, 20min-1h30 endurance, 40-55km/h)

Fully custom UAV for aerial photography, not available as a kit or in any other form.

(a joint development of AerialRobotics and TriggerComposites, www.pteryx.eu)

All can be hand-launched, however for Pteryx with over 4kg AUW

it is possible to use bungee launch if you want to operate reliably

even under stress.