Help Me Buy The Right Silencer

Question about the apexi ATS silencer, can't figure out which size to get. When I purchased my 96 GSR it already had the apexi N1 system installed but the silencer was lost. Now I need this car for work and it needs to be much quieter. When i look into the silencer i see a 90mm option for NA N1, or a 115mm option for GT spec or N1 Turbo. i honestly have no idea which model i have, is there any way i can find out? Do i measure at the exhaust tip?