Lawrence Levine wrote a 31,594-word palindromic novel titled Dr. Awkward & Olson in Oslo.
There are many long sentences possible if you just include a lot of commas. The extreme came from Peter Norvig who designed a computer program that could automatically generate as long a palindrome as possible using the "A man, a plan, a canal, Panama" as the base. Norvigs program created a 21,012-word palindrome.