China has three major islands Fiery Cross, Subi, and Mischief Reef which all have approximately 10,000 foot runways, deep water harbors, and enough reinforced hangars to house 24 fighters as well as bombers, tankers, and airborne early warning aircraft. China is also building other airfield support facilities.
Three air regiments on the three islands would add up to a fighter division, a formation consisting of about 17,000 personnel.
China can load up the islands with various missiles and defense batteries. They would have an interlocking and mutually-supporting SAM umbrella over most of the Spratlys, as well as ASCM coverage over the heart of the South China Sea. China would be able to strike, with either DF-21C land-attack ballistic missiles or CJ-10 cruise missiles, U.S. and allied facilities and airfields throughout the Philippines and even to Singapore.
China could also build a large island base at Scarborough Shoal.
Fiery Cross is hardly a little runway perched on a reef and is roughly the size of a typical mainland fighter base and it has a sizable harbor. Subi Reef is almost 50 percent larger again in area, and contains a massive enclosed deep-water harbor over two miles wide. A visual comparison of Subi Reef with Pearl Harbor shows what a huge installation the Chinese have created from scratch.
Mischief Island is even bigger and comparable to the size of DC.
China's missiles are mobile systems and would be able to move and hide an sizable islands.
China can also bury and harden facilities on the islands.
With enough combat power brought to bear, China’s new island bases would be vulnerable to being overwhelmed.
China’s goal may be to increase the costs beyond the level that the US would be willing to commit.
SOURCES- War on the Rocks