I still like the definition another website uses, that LF starts at 100 square centimeters. That includes 9x12cm, but excludes 6.5x9cm.
By stretching that definition a little, ULF starts at 600 square centimeters. 8x10" is 500 cm^2, so falls below that limit. 24x30cm (9.5x12") is 720 cm^2, and is above the limit.