I have a large left popliteal aneurysm. When should I consider surgery?

Soon. If you have a large aneurysm in the popliteal artery, it is at risk for thrombosing (clogging off) or embolizing (tossing debris), and puts you at risk for losing your leg. Larger aneurysms can also compress local structures and cause blood clots in your vein, or pain from pressure on the nerve. It is also possible for the aneurysm to rupture, but this is more common in aortic aneurysm.
Soon. Popliteal aneurysms rarely rupture but clot threatening limb ( leg & foot ) amputation, material inside can break off & block arteries "downstream", compress nerves or veins causing damage. When symptomtomatic, the risk of amputation is hign. Bending & straightening the leg at the knee "bends" the aneurysm making these compications more likely. Surgery if > 2 CM or have symptoms.