Using BLE scanners apps (in my Android mobile phone) like LightBlue and nRF Connect acting as central device to scan and connect with my SoC Arduino UNO R4 WiFi acting as a peripheral device, it's device name are truncated down to 20 chars.

So, I'd use 20 chars at maximum size.

See also: https://community.particle.io/t/ble-setdevicename-only-realistically-works-for-14-or-fewer-characters/66329