I'm no expert but I did spend about 3 weeks in Costa Rica during the month of December, that time of the year is the dry season so you could expect small surf (except a small possibility of large surf at the rare North facing spots) and clean offshore conditions all day, when I was there the waves ranged from knee to shoulder high always super clean offshore and mostly pretty perfect little walls,

I would say the smaller board would be the best option but I would make sure your proficient on it before you go, you don't want to waste good holiday surfing time struggling with a board that is difficult to ride