I don't think one can fully answer to such a broad question. It depends on the population, the environment (presence of predators for example), the genetic background (having a big stomach is good but only if the intestin is big too), the population's social relations, .. Many traits are correlated and one might want to say that a given trait is beneficial but it is not because this trait is correlated (because of genetic, physical or social constraints). Here is a list of possible advantages/disadvantages.
- visibility to predators
- ability to hunt bigger preys or to manipulate smaller preys
- running faster
- be more impressive
- fighting ability
- energy consumption for maintaining a body
- Quantity/quality of food to be found
- homothermy (heat loss)
- sustaining its own weight
- Rooms for the organs
- Attractiveness to the other sex
- being more able to move freely (in viscous environment (water))
- mechansisms for bringing nutrients and air to the tissues
- density of the body (water)
- excretion of wastes
- moving in small habitats
- Hiding in small areas
- bearing big and complex structures
- Parental care
- flying better
You might be interested in life-history traits, to allometric relations. Maybe you can be interested by this article as well.
You might also be interested in the philosophical discussion of adaptation, optimality, cost and benefit like this book (which I haven't read!) for example.
Concerning the relation between heat loss and size, you might be willing to have a look to the Kleiber's law. This law has been extensively debated and is today thought (or known) to be wrong.