Q: Construct the longest coherent sentence you can such that the nth word is n letters long.

A:

Such sentences are also called "rhopalic."

I do not know where family doctors acquired illegibly perplexing handwriting; nevertheless, extraordinary pharmaceutical intellectuality, counterbalancing indecipherability, transcendentalizes intercommunications' incomprehensibleness.

From "Time" magazine, January 10, 1977
O to see Man's stern poetic thought publicly expanding recklessly imaginative mathematical inventiveness, openmindedness unconditionally superfecundating nonantagonistical, hypersophisticated, interdenominational interpenetrabilities.

If you add the condition that the letters in each word must be in

inverse alphabetic order, you have
A zo fed upon solid toffee zyxomma.

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