Kansas City is also a contendor, while the viewing isn't as good as Sedgwick County, it is much larger (Over 4x as large). Here are the figures (approxamently)
Kansas City
2955.13 m²
0.00 km²
0.73 acres
31808.74 feet²
Sedgwick County
790.72 m²
0.00 km²
0.20 acres
8511.23 feet²
@KCZooFan: thanks for posting all of those dimensions, and I'm a little surprised at just how large the San Diego Safari Park and Kansas City lion habitat are. Sedgwick County's is amazing, as are many others scattered at zoos in the United States. In a way the many gorgeous exhibits are somewhat unneeded due to the fact that lions spend a huge portion of their days simply sleeping. Also, since 2004, large lion exhibits (perhaps not quite as impressive as the ones mentioned here) have been built in Denver and Oregon.
I do not have the actual numbers, but San Diego (assuming you mean the zoo) is atrociously small. Many people have pointed out on the heated Elephant Odyssey threads that it is way too small for lions. Even the few people who actually like EO agree on this.
Philadelphia is average sized - pretty nicely designed, but nothing really big or special.
Very cost effective, I actually liked that when l visited the male was positioned on a high rock that could be seen from a distance, with the appearance of being free roaming! To enhance the experience he was roaring away like the king he is!