So I don't know much about how helpful each of these ships are in Mass Effect, but I do know we want more. Any ideas on what we want?
Overview first. Your PC's analysis of what you want will come with Turn 1.
Corvettes are a minor part of the Mass Effect lore, one which typically gets very little attention. They're ten-crew craft used for combat and survey. Picture sci-fi staples like the Millenium Falcon or the Ebon Hawk, for scale. I'm defining them as tending to do well in picket ship and scouting roles. Good for screening larger vessels, but not much heavy armament of their own. That said, they're very maneuverable, cheap to make, and in lots of numbers can dump enough cheap munitions downrange to threaten larger vessels.
Frigates are destroyers -- screens for the larger vessels which can also threaten larger ships in wolf packs of four to six thanks to high maneuverability and the ability to carry effective torpedoes. That said, they are flimsy in comparison to most ships. They are often assigned to more hazardous scouting and survey work due to having greater firepower than corvettes while still being able to land on planets. They are also often used to screen larger vessels in fleet engagements. The Normandy SR-1 was a frigate. They get up to 100 meters in length.
Light cruisers are the earliest that you can start defining vessels as ships of the line. They're the smallest vessels where it's practical to mount a spinal cannon, and thus get a big jump in firepower. They're relatively fast, can maneuver well, and are more heavily armored than frigates, to the point where they can stand up to heavy blows to a limited extent. They're the infantry of most fleets due to being very cost-effective. Light cruisers are the flagships of typical peacetime patrol missions, and form the flanks of the battle line in serious fleet engagements. They can land on planets, but only very low-gravity ones. Light cruisers are typically 300 to 400 meters in length. The Normandy SR-2 straddled the line between frigates and light cruisers.
Heavy cruisers fill the same doctrinal role as light cruisers, and are in general larger and tougher in every way. Heavy cruisers are generally what you see in a situation that calls for heavy firepower but doesn't demand a dreadnought. Most planetary fleets will have at most heavy cruisers, and will make them their flagships. They are slower than light cruisers and can be outmaneuvered if supported improperly. It's a very, very rare planet that a heavy cruiser can land on and take off from again, but they do exist. There are no clear canonical examples of heavy cruisers, but the smaller Reapers may qualify. Heavy cruisers are typically 500 to 700 meters in length.
Dreadnoughts are the largest and most powerful ships in existence. 800 meters to a kilometer in length, they possess unmatched firepower, armor, and shielding. Dreadnoughts are strategic weapons and the premier space combat vessels. Nothing can stand up to them in a stand-up fight save another dreadnought. They allow navies to engage at unmatched range and with incredible power. That said, they are very slow and their firepower depends on aligning their spinal cannons with their target, making them vulnerable to swarms of large craft that get close. They demand a comprehensive screen of smaller escorting vessels. Dreadnoughts cannot land on planets at all; not if they want to come back up. At present, neither the canonical
Treaty of Farixen nor the canonical
Citadel Conventions exist, and there is thus no restriction on the use or construction of dreadnoughts. That said, few polities are capable of affording them. The Destiny Ascension was a dreadnought, and the largest of Reapers like Sovereign and Harbinger, at over double the typical size, are examples of super-dreadnoughts.
Carriers exist in the canonical games, but are only invented by humanity. They do not yet exist in this quest. Fighter craft are similarly marginalized; they have no role in void naval warfare.
I have to write you an update. 
