This is way cool. You're basically just extending the words' length the longer you get, right? And variate it a bit. Mostly it was the words that were new to me or words I don't normally would use that were the hardest to spell, it would be cool if you ever made a new one to have many more factors play into how it finds the words, such as the amount of vowels vs. consonants in a sentence, the harder to "pronnounce" the harder I imagine it being to spell in your head
I looked online and found a word list containing about 60000 words, one word on each line. I used visual studio to do a huge Find-Replace to format the words into an array so that it can be accessed just like you would out of a normal array, and picked random words out of the array.