What is the longest word in the english language?

[Mithrandir]

So yes, you can make it as long as you want (arbitrarily long in fact, at least as a "theoretical" compound), but for "real" proteins, the longest I can think of is the cardiac variant of Titin, with a staggering length of 26,926 aminoacids (if you then take an average of, say, 8 letters when constructing the chemical composition the "hard way", it comes with over 215,000 characters, what a word!),
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Titin is indeed the longest protein, but if you want to get really out of control, think about the proteoglycan Hyaluronate. Mr = 2*10^8 - it's the size of a small bacterial cell. Best of luck to anyone wishing to name all the individual parts of that badboy.

The fact that Hyaluronate is also one of the most stable molecules in the body just goes to show how amazing evolution is.