Do you want to mean that the maximum die size that is possible in lower technology nodes is 20 x 20 mm2.Without "reticle stitching" you are limited to the reticle dimension (as scaled by the optics, commonly 10X) which tends to be about 20mm. I have seen some cases where a 20mm field has to be reduced for depth-of-field / astigmatism litho issues, the physical field on-wafer is a best case.