Please note that the tri clamp size is determined by the outside diameter (OD) of the tubing, NOT the OD of the flange. You can refer to the below quick list for tri clamp sizing with corresponding flange OD to confirm the size of the Tri Clamp.
1.5 inch tri clamp, OD of Flange=50.5mm
2 inch tri clamp, OD of Flange=64mm
2.5 inch tri clamp, OD of Flange=77.5mm
3 inch tri clamp, OD of flange=91mm
4 inch tri clamp, OD of flange=119mm