ship sizes , the bigger, the greater ?

[trooper]

1) Warships : When you discover a new ship size, do you systematically make your smaller hulls designs obsolete ? Is there a reason to make bigger ships escorted by small ones (except for fun ?).
The maintenance/fire power balance is better with big ships, isn't it.

2) Non-warship hulls : Are they really usefull ? a battlecruiser sized ship can make a good, large and fast transport or colony ship, or an efficient carrier or mine layer, without the limitations (speed, cargos, weapons) of specialized hulls...