Well, it's a little weird they left out the "sky" if they meant "big sky country". There is an area in central Texas that is referred to as Big Country, but I'd like to think they wouldn't have two in Texas.
As far as the clue about #17 being near a major metropolitan area, that could also be a Texas reference (though I doubt it is). Texans refer to the Dallas-Forth Worth area as the Metroplex. Another possibility is that #17 could have something to do with the NY Metropolitans aka the NY Mets. Or it could be at somewhere like the Metropolitan Museum of Art.
By definition, a metropolitan area is somewhere with more than 50,000 residents. Here is a list of the "major" metropolitan areas:
1) New York-Newark-Bridgeport, NY-NJ-CT-PA - 21,976,224
2) Los Angeles-Long Beach-Riverside, CA - 17,775,984
3) Chicago-Naperville-Michigan City, IL-IN-WI - 9,725,317
4) Washington-Baltimore-Northern Virginia, DC-MD-VA-WV - 8,211,213
5) Boston-Worcester-Manchester, MA-RI-NH - 7,465,634
6) San Jose-San Francisco-Oakland, CA - 7,228,948
7) Philadelphia-Camden-Vineland, PA-NJ-DE-MD - 6,382,714
8) Dallas-Fort Worth, TX - 6,359,758
9) Houston-Baytown-Huntsville, TX - 5,641,077
10) Atlanta-Sandy Springs-Gainesville, GA-AL - 5,478,667
11) Miami-Fort Lauderdale-Miami Beach, FL - 5,463,857
12) Detroit-Warren-Flint, MI - 5,410,014
13) Phoenix-Mesa-Scottsdale, AZ - 4,039,182
14) Seattle-Tacoma-Olympia, WA - 3,876,211
15) Minneapolis-St. Paul-St. Cloud, MN-WI - 3,502,891
16) Denver-Aurora-Boulder, CO - 2,927,911
17) San Diego-Carlsbad-San Marcos, CA - 2,941,454
18) Cleveland-Akron-Elyria, OH - 2,917,801
19) St. Louis-St. Charles-Farmington, MO-IL - 2,858,549
20) Tampa-St. Petersburg-Clearwater, FL - 2,697,731
21) Pittsburgh-New Castle, PA - 2,462,571
22) Sacramento--Arden-Arcade--Truckee, CA-NV - 2,211,790
23) Charlotte-Gastonia-Salisbury, NC-SC - 2,191,604
24) Portland-Vancouver-Beaverton, OR-WA - 2,137,565
25) Cincinnati-Middletown-Wilmington, OH-KY-IN - 2,147,617
26) Orlando-The Villages, FL - 2,053,623
27) Kansas City-Overland Park-Kansas City, MO-KS - 2,034,796
28) Indianapolis-Anderson-Columbus, IN - 1,984,644
29) Columbus-Marion-Chillicothe, OH - 1,953,575
30) San Antonio, TX - 1,942,217