To build on this and give more examples, the largest telescope is the FAST ratio telescope, with a 500m diameter. The largest fully steerable telescope is the Green Bank Telescope with 100m diameter.

But both of those are radio telescopes, so a more fair comparison with JWST would be the sizes of optical/near-infrared/mid-infrared telescopes. The Gran Telescopio Canarias has an 10.4m diameter primary that is made of a number of smaller mirrors. Keck's design is the similar (and came before the GTC), though the primary diameter is slightly smaller at 10m. The Large Binocular Telescope (LBT) has two 8.4m diameter mirrors. All three of these telescopes are capable of observing in the mid-infrared; in principle with higher angular resolution but less stable point spread functions and reduced sensitivity compared to JWST.

The Vera Rubin Observatory mirror is about the same diameter as the LBT's mirrors, but the large secondary and tertiary mirrors reduce the collecting area to be equivalent to that of a ~6.7m mirror.