A pedal wrench is 15 mm. If your pedals are tight, you will need a wrench that is strong enough to do the job and allows you to develop enough leverage... Regular mechanics tools like adjustable wrenches and regular wrenches are often too wide (thick) to fit. A 15mm cone wrench will fit but is easily bent out of shape if the pedal is tight.

Make sure you are turning the pedal axle in the correct direction to take the pedal off. When facing the bike, put the wrench on the pedal axle and turn it towards the back of the bike. This rule works on both the left and right side of the bike.

I use the pedals that are clipless SPD on one side and regular platform on the other. I seldom use the platform side, but it is nice to have when I'm not wearing my cycling shoes.