What is the largest size of visible glass that can be put into a fire-rated door?

It depends on the rating. All glass used in fire-rated doors must be listed glass and be either ¾” wire or other listed material. Basic guidelines:

• 20 minute fire-rated door – 1,296 sq. in. per lite
• 3/4 hour fire-rated door – 1,296 sq. in. per lite
• 1-1/2 hour fire-rated door – 100 sq. in. per lite
• 3-hour flush door, no glass – per manuf. requirements​