I want to put a FC in a sailplane and basically I just need Stabilize and RTL flight modes. Any suggestions for a reliable and small FC (I am used to Pixhawk 2.1 which is way too big in this case) ? I need GNSS and airspeed, OSD would be nice but not necessary.
I flew a sailplane (Siren) with a LibrePilot Revolution (4cm x 4cm, Revolution Nano is even smaller) running ArduPilot. Back then it still had limited features, but maybe enough for what you say you need (here). Note that you will have to add the GPS if you want RTL (not needed for just stabilise), and I’m not sure it can handle the airspeed (maybe compatibility has grown, I flew the first version compatible and it still had some sharp corners to round).
However, if you fly with FrSky you can have all the telemetry (vario included) just by adding the right cable (basic here, instrumental flight here)