9-266Q is the largest system in the CCP EVE online database export found here. It is 203,0220982 AU in diameter. Although SQL doesn't work for me right now the longest jump in that system is said to be between two gates at 273 AU after checking a few forums. The diameter of 203 AU must then be drawn between planets because it's less than the max jump between gates. So the largest system diameter in the database does possible not provide the largest warp within a system in game. Although I read that the database got coordinates for gates and planets so you should be able to calculate distances between each object in let's say, the 5 largest systems, and get a more proven answer.

Celestials also include Corpse of human and cargo container making it possible to create arbitrary warp distances. But I guess you meant stuff that can't be moved like stations, gates and planets? Otherwise you could take the system with the largest diameter between gates/planets (not same as the above diameter of 203 AU) and add 20 AU (limit for stuff at least) outside that circle, dump two cargo containers opposite each other and warp between. The furthest bookmark created from star at the time of the limiting deep safe spot range was 5,9 billion AU (like the milky way).