Arduino pro mini + ws2812b

Size ? the crystal that's on mine is also tiny though.

I was going to say, 'didn't the Arduino come with some male header pins ?' Soldering wires and sorts onto the unit is anyway not really the way to go. Build a shield and insert the Arduino. But this does require more parts and therefore more money.

But the Crystal also needs trimming capacitors, and they are both on his and on my board on the opposite side of the MCU. Would a resonator unit need those as well ?

The UNO doesn't have them. But it does have a 1M parallel resistor, which is one part.

AFAIK, most resonators include built-in capactitors matching the resonator characteristics and thus have three terminals.