I see there's GSI which seems to have decent volume but then there's also ZI which has like no volume. They both say 5,000 oz Silver. And then I see a few others in smaller amounts (2,500 oz for QI, 1,000 oz for YI) but those have no volume, either.
The most active silver contract is the "old" (5,000 ounce) COMEX contract which trades on the CME platform. There can still be other 1000 ounce and 5000 ounce contracts that were originally listed at the CBOT that I believe are now listed on ICE or LIFFE.
So the SI contract on CME Globex is the most active silver futures contract in the world? http://www.cmegroup.com/trading/metals/precious/silver.html