Antenna for ESP32-pico-d4
here one says that he literally just connected the antenna to ground and LNA_IN would that work?


Yes. LNA_IN is the input to the Low Noise Amplifier - the input stage of the receiver / output stage of the transmitter.


  and which antennas would you recommend? (as small as possible is only gonna be used for BLE and)


You want one tuned for 2.4GHz. The simplest is a quarter wave dipole at 3.125cm in length. Better would be a proper 2.4GHz antenna of the kind you get on a WiFi router or some USB WiFi dongles.