There were several spell checkers for the TRS-80 model 100, which maxed out at 32K of RAM. More typically, it has 24K, because the weird RAM chips were so expensive.
Plus, few people had floor drives for the 100 —you had to operate from a cassette tape.
I was going to point out my Panasonic W1030 word processor as probably even more impressive since it's not even really a PC.
But then I found specs and its dictionary uses 384kB [1]. Then again it's from the latter 80s, maybe previous word processors attempted more constrained spellcheckers.
Plus, few people had floor drives for the 100 —you had to operate from a cassette tape.