Though most large creatures have a face of 10 feet, the info is always included for each individual creature. This info is not included with the Half-Ogre, so he has a regular facing of 5 feet.
Dodging leaves would require an Escape Artist check against DC 15 once as long as you're not taking special activities, such as combat. In combat, it requires a check each round. Taking 10 is permitted. You're getting a +4 circumstance modifier for each step smaller than Medium, which can be negative, so ben get's a -4. Getting in touch with a leaf wouldn't automatically get you in touch with worms. Movement is halved for anyone trying to dodge leaves. You cannot dodge leaves when attacking or running.