Does such a thing actually exist or is it seller misinformation on Amazon?

There are 11W 9" T5's, same size as a 6W - sort of HO version. Never seen any length between 8W 12" and 13W 21"

Do you mean the Osram T2 (6mm) display cabinet lamp? Made in 6W, 8W, 11W, 13W; the 11W lamp was 422mm.

Do you mean the Osram T2 (6mm) display cabinet lamp? Made in 6W, 8W, 11W, 13W; the 11W lamp was 422mm.

No, it was T5. Same diameter as the 4, 6, and 8 watt versions.

Well, the 17" would correspond to 11W (when interpolating the lengths of all the common T5 miniature wattages), so it is clearly not just one wrong number.

There were also T4 lamps with similar wattages as the T5 miniatures, but they were slightly longer (about 1" longer for the 8W; and a bit different electrical specs - a bit lower current/higher voltage), but there I would expect the 11W to be still a bit longer (at least 18").