The hold of holding is actually a knocker that one bolts on a door. Once bolted onto the door, the number of knocks given along with the command word spoken the hold of holding enchants the door to become a portal to an extradimensional space.

The size of the room depends on the power of the enchantment, and each tonnage is equal to 100 cubic yards.

Tonnage   Minimum of Cargo Area            Caster Level     Market Price

1 ton     20 ft. by 15 ft./10 ft. height   1st level        14,000 gp

2 tons    20 ft. by 30 ft./10 ft. height   2nd level        28,000 gp

3 tons    40 ft. by 25 ft./10 ft. height   3rd level        42,000 gp

5 tons    50 ft. by 30 ft./10 ft. height   5th level        70,000 gp

7 tons    50 ft. by 40 ft./10 ft. height   7th level        98,000 gp

9 tons    50 ft. by 50 ft./10 ft. height   9th level        126,000 gp

While all the holds listed here take a minimum spellcaster of 13th level some really large holds would take much more.

Caster Level: 13th; Prerequisite: Craft Wonderous Item, mordenkainen’s magnificent mansion, creator’s class level must be a equal to the tonnage of hold; Weight: 5 lbs.