Which side has the leaner, meaner fighting machine?
Without knowing anything about either country (other than long-past history), I would guess it’s Nicaragua.
I don’t think Costa Rica has an army?
Without outside help, Costa Rica doesn’t have a chance.
The entire region is pro-communist, pro-Ortega, pro-Chavez, pro-Morales.
Brazil just elected a communist terrorist, Dilma Rousseau. The only remaining anti-communist country with the military might to push the Nicaraguan army back up the river is Columbia.
Given that the US sides with the communists these days, Columbia will go it alone, and probably have to contend with opposition from the US and Brazil.
If the Nicaraguan communists wanted it, they could take all of Costa Rica. I doubt anybody would do anything about it.