According to most scales, Medium is 55.5-59cm and Large is 59-62.5cm.
Will I be good with a Medium, and it wont be too tight on my head? Or should I go with a large, just incase?
Don't buy a helmet without trying it on. If you don't have the opportunity to try a helmet (no local shops around, etc..) then buy something with adjustable dial in the back of the helmet. They work pretty well.