I have just had a look at my catalogue and the seal sizes listed for KX500s are as follows...
83-87, 43x55x10.5 (right way up forks)
88, 43x55x9.5/10 (right way up forks)
89, 45x58x10.5/11 (right way up forks)
90, 41x53x8/9.5 (upside down forks)
91-96, 43x55x9.5/10 (upside down forks)
97-03*, 46x58x9.5/10 (upside down forks).
So, the 83-87 bikes ran 43 mm right way up forks.
88 bikes ran 43mm right way up forks with different seals.
89 ran 46mm right way up forks.
90 bikes had 41mm upside down forks.
91-93 bikes had 43mm forks.
97-03* bikes had 46mm forks.
*I would guess that the 04 bikes also use the same size forks as the 03 but don't know for a fact.