O. briareus has a body about 5 inches long and very long arms, up to two feet long. For this reason, it might be a bit cramped in a 50 gallon tank - 75 would be more comfortable. They live about a year.
O. vulgaris is even larger. with an 8 inch body and arms to over 30 inches. It needs quite a large tank- maybe 125 gallons.
On the other hand, O. joubini and similar dwarf octopuses can live in a smaller tank - 30 gallons. They don't live as long as the larger octopuses.