I use the smaller hull sizes for boarding ships, troop transports, point defense ships and minesweepers.  Non-combat ships use the size best suited to their role.  For instance a ship used to drop spy satellites rarely needs to be larger than an escort.
Regarding the front line ships:
- Boarding ships:  I find a light cruiser to be just the right size.  Allows a couple shield depleters, a shield and a couple boarding parties.  No need for more.
 
- Troop transports:  Again the light cruiser.  In some mods (ie: Proportions) you do need the huge transport but in most mods 1200 kt cargo space is plenty.
 
- Point defense:  I usually use one or two sizes smaller than my largest ship.  Rather than have the PD concentrated in one spot, I like to spread the guns around.
 
- Minesweepers:  A single base ship minesweeper is all you need, but lose that and you have none at all!  I prefer several smaller ships and even then carry 50% extra in a fleet.  The loss of one or two is not critical, just annoying. 
 
To blockade a planet I find it safest to use unarmed ships.