Here are some of the specifications of the USS Remora. Now the USS Remora did undergo some modifications and improvements over her time of service and You can have a look at the differences here:

Initial construction and release in 1945.

Displacement: 1,570 tons (1,595 t) surfaced
2,414 tons (2,453 t) submerged
Length: 311 ft 8 in (95.00 m)
Beam: 27 ft 4 in (8.33 m)
Draft: 17 ft (5.2 m) maximum

Guppy II (first modification 1947)

Displacement: 1,870 tons (1,900 t) surfaced
2,440 tons (2,480 t) submerged
Length: 307 ft (93.6 m)
Beam: 27 ft 4 in (7.4 m)
Draft: 17 ft (5.2 m)

Guppy III (Second modification 1961)

Displacement: 1,870 tons (1,900 t) surfaced
2,440 tons (2,480 t) submerged
Length: 307 ft (93.6 m)
Beam: 27 ft 4 in (7.4 m)
Draft: 17 ft (5.2 m)

As big as this sounds, in comparison it is actually not that big. The comparative submarine being the world’s largest is 175 metres long. Almost double as long as the USS Remora.

Either way however there is little room inside no matter how big it is overall. All quarters of a submarine are small, including your sleeping, operations, mechanics and more. You can expect tight spaces, that require some agility and patience. You certainly would not be able to host a party in a submarine, complete with people, food, drinks, a DJ, a photo booth perhaps, decorations and more.

You would not be able to down grade that to a small gathering either. Subs were never built for comfort they were built for ability. A machine built solely for work and its purpose as an underwater vehicle with missions/tasks backed with purpose.