From Lexicanum

The Emissary is the first vessel created for the renovation of the Tau Navy. Designed primary for transport for Water Caste dignitaries, Tau commanders and Ethereals and second for battle. They are also used as large merchant vessels. Despite its size, the Emissary is well equipped and perfectly capable of looking after itself against all but the largest capital ships.

These ships are now becoming common within the Tau Empire. As this class was the first of the renovation program and their versatile role, their number may increase in the Kor'vattra.

The size of the Emissary is roughly the 1/4 of an Imperial Emperor Class Battleship (as seen in Fire Warrior game). It can transport up to 2 squadrons of Barracuda.

Sources

Imperial Armour Volume Three - The Taros Campaign

Kor'vattra - Tau Navy

Type	Design	Class
Capital Ships	Tau Battleships	Gal'leath - Explorer • Or'es El'leath - Custodian
	Cruisers	Il'fannor - Merchant • Lar'shi - Hero • Il'Pourri - Emissary • Lar'shi'vre - Protector
Tau Escorts	several classes	Kir'qath - Defender • Skether'qan - Messenger • Kass'l - Orca Gunship • Kir'la - Warden Carrier Escort • Kir'shasvre - Castellan Heavy Escort
small craft	air/space bombers and fighters	Manta Class Bomber • Barracuda Class Fighter • Remora Drone Stealth Fighter‎
mercenaries and allies	Kroot vessels	Warsphere
	Nicassar vessels	Dhow
	Demiurg vessels	Stronghold Class Commerce Vessel • Bastion Class Commerce Vessel