Some where 10 mm. What ever size it is find a good fitting hex key and just turn it the proper direction. Been a while since I did that, but I think counter-clockwise on the left and clockwise on the right to loosen. Someone else will be fast to correct me if I'm wrong!.

I'm pretty sure that is a self extracting crank arm. What's the exact model/part number? That'll probably be on the inboard side around the hole for the pedal.

Here are your pic's.....




https://www.bikeforums.net/g/picture/33025983