budejovice: , the 179' patrol class was the smallest ship that the Navy used by itself for travelling across open ocean. Everything smaller (landing craft, etc) crossed oceans on other ships.
Lenght is but a part of the equation. They had a draft of 10 fr, a complement of 60 men or so and had to carry a gun, anti-aircraft guns, depth charges. Warship typically have (much) more crew onboard than civilina watch. Simply put, they need to carry at least 3 times the crew ordinary used, as to be fully operational 24/7. So, extended sea service in such ships was exhausting.
Corvettes assigned to the Atlantic duty were a bit longer ( 62,5 m ), had more draft , but 90 men on board. Wheather on the northern convoy routes ( Halifax - Northern Ireland) was at best a challenge, but Arctic duty (Murmansk bound convoys ) was nightmarish. The old battleships assigned to convoy escort suffered badly, and they were large, with considerable draft and somewhat spacious. ) Just imagine what was life then on the corvettes (60 or 80 degree rolls were not uncomon ( meaning the ship sailed alternating a 40° inclination on each board ) or even on destroyer escorts.
Still, sub-chasers or corvettes were small enough to be produced in small civilian shipyards ( used to producing trawlers ) , were cheap, and thus made up half of the escort capabilities.