It is rather difficult to determine the biggest chamber. First it's a problem how to measure its dimensions, and then it's a problem to decide which aspect of the survey data is most significant.
How do you measure the size of an underground chamber? In caves, it is rather difficult to determine exact volumes, as surveying just follows a single line. You just get a sort of skeleton of the cave. The solution to measure surface and volume is to add the distance to walls, floor and ceiling. You measure to the wall as often as possible. In big chambers you may survey once along the wall. The height is determined using a thin rope and a gas filled balloon. This is also done as often as possible. The result is nevertheless very inaccurate concerning volume, but length and width are quite accurate.
As a result it is actually impossible to make a sort of ranking based on survey data. While volume would be the best criteria for this, it is also the least accurate number. This changed in recent years with the introduction of 3-D laser scanners. They allow the scan of all walls and ceiling and the construction of a computer model. Unfortunately the technology is very expensive and the survey is a lot of work. While currently only a dozen caves was scanned so far, this might become the main surveying technique for such huge caverns in a few years.
The official list is linked below. Our topic is tourist sites, and so we made an excerpt. The two biggest chambers plus all which are accessible in any way. We were astonished how many on Bob's list with 38 chambers were actually show caves.
|Sarawak Chamber||Good Luck Cave||Gunung Mulu National Park, Sarawak, Malaysia.||641 m||429 m||113 m||154,530 m²||9,670,000 m³||This is the biggest cavern (the biggest single chamber) of the world, accessible only to cavers|
|Miao Room||Gebihe system||China||824 m||336 m||177 m||140,540 m²||10,570,000 m³||This is also the biggest cavern (the biggest single chamber) of the world, accessible only to cavers|
|main passage||Phong Nha Cave||Phong Nha-Ke Bang National Park, Quang Binh Province, Vietnam||3,100 m||80 m||180 m||m²||m³||This is also the biggest cavern (the biggest cave passage) of the world, accessible on cave trekking trips|
|Majlis Al-Jin||Majlis Al-Jin||Oman||320 m||225 m||120 m||61,000 m²||4,090,000 m³||A karstfenster in the middle of a huge chamber, can be seen from the rim.|
|Gouffre de la Pierre Saint Martin||France||255 m||245 m||199 m||43,000 m²||3,650,000 m³||Easily accessible through a tunnel, but no further development and no light.|
|Salon de Gigantes||Grutas de Bustamante||México||39,730 m²||show cave|
|main passage||Gua Payau (Deer Cave)||Gunung Mulu National Park, Sarawak, Malaysia.||870 m||177 m||270 m||38,600 m²||6,310,000 m³||show cave|
|main passage||Kocaín Mağarası||Mediterranean Region, Turkey||m||m||m||37,200 m²||m³||not developed but easy to visit|
|Big Room||Carlsbad Caverns||Carlsbad Caverns National Park, New Mexico, U.S.A.||1,200 m||190 m||78 m||33,210 m²||m³||show cave|
|main chamber||Grotta Gigante||240 m||138 m||132 m||33,120 m²||m³||show cave|
|Martel's Chamber||Skocjanske Jame (Skocjan Caves)||Slovenia||308 m||123 m||146 m||32,090 m²||2,200,000 m³||not part of the show cave, but the show cave is only slightly smaller|
|main chamber/passage||Phu Pha Petch Cave||Satun GeoPark, Satun Province, Thailand||623 m||150 m||58 m||30,500 m²||m³||show cave|