There are two degrees of sea anchor due to loose terminology.
First is a sea brake designed to control boat speed when trawled behind the boat. It improves ride and safety
in heavy seas.
Second is to lock the boat on the water
,usually deployed over the bows and in a lee shore situation, again in heavy seas where sailing or motoring out of the situation is difficult or impossible.
There's lots on this site about 'Drogues' and 'Sea Anchors'. Find a manufacturer, give them the boat size and type and you'll get their conservative estimate.