My pedal just unscrew. It has a hexagon-headed screw with '10.9' screw label. Anybody knows what size of wrench I need for this screw?
Common sizes are a 5 or 6 millimetre Allen key or Hex Driver on the back side of the crank. Or, you can also use a 15mm pedal spanner on the two flats which will be on the pedal-side of the thread. These tend to be a bit thinner than a normal adjustable spanner.
Do be aware your left foot pedal is a left hand thread so you turn it the opposite way to tighten than what you would expect. The rule is that tightening the pedal will tighten the chain if you don't stop the crank rotating.
Also consider why did it unscrew in the first place? Was it never tight?
I've recently found clipless pedals with either 8mm or 10mm hex sockets on the end. That's a hefty-sized hex key, too big to carry on the bike for regular use.