8" diameter sphere should be more than enough

A cubic foot of air has volume of 1728 cubic inches.
A 8” diameter sphere has a volume of 268 cubic inches.

A cubic foot of air displaces ~64 lbs of seawater.

The 8” diameter sphere has about 15% the volume of the cubic foot so it displaces ~9.5 lbs.

When the sub is in the water it only weighs 15% of its outside the water weight. If the sub weighs 50 pounds in the air it only weighs 7.5 lbs in the water, under the 9.5 lbs of seawater displaced, so it will be positively buoyant.