I actually need to find the seatpost diameter... I need to lift the saddle a bit more, and this tube is already at it's limit. I'll go to a shop and ask for assistance, as I don't have the tool to measure with precision.

I took it out for a first quick ride, around 30km and besides the knee issues due to the lower saddle, it's a very comfortable ride.I may buy larger tires, 25C probably (now I have 20C and it's just too thin)