Detection of the presence of 230V AC mains voltage - digital signal for the microcontroller

2 resistors as voltage divider, diode, capacitor. In SMD some tiny thing will take up as much as the optocoupler itself. Unless you need isolation from the mains, then no. Perhaps 2 transistors in a darlington circuit would be enough, the gate hanging "in the air" and close to the 230VAC path and it will also work. The circuit is basically the same as in voltage detectors.
And if you have more current in the 230V circuit then take a reed switch, wrap it a few times with one wire from the power wire and connect it to the uC. The current in the wire will trigger the reed switch.
Other than that, it is the optocoupler that leads the way.

Krzysztof Kamienski wrote:

that is, how to correctly connect the LED to the mains, i.e. the optocoupler too.

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True, I didn't take into account that the components powering the optocoupler also consume some space.