А вы как думаете?

Кому не лень, пожалуйста, дайте ссылки на грамматическое правило, обосновывающее правомочность варианта 5 foot 6 (если вы считаете его правомочным).

I'm five foot six - правильно.
I'm five feet six - НЕправильно.

Usage Note & Our Living Language

tumanov, спасибо за ссылку.

п. (1) - это понятно. Я тоже приводил соответствующую цитату единственного случая, когда используется единственное число единицы измерения:
http://grammar.quickanddirtytips.com/units-of-measure.aspx
"For the next point, let's go back to talking about feet. The singular is obviously foot, and the plural is usually feet, except when you are using it as a compound modifier. You would say that Squiggly climbed a tree that was 10 feet tall†; feet is plural there. But you don't say Squiggly ran up a 10-feet tree—you say Squiggly ran up a 10-foot tree. Foot is singular when it's part of a compound modifier. That's also true for other units of measure like inches and pounds. Squiggly was running from a 100-pound monster, and he nearly lost his 10-inch tentacles."

Вот ещё, более краткая заметка:
http://www.merriam-webster.com/dictionary/foot
- plural foot used between a number and a noun:
- plural feet or foot used between a number and an adjective: <6 feet tall>

п. (2) Обычно используется нормальное множественное число для всех единиц измерения: "6 feet", "6 miles", "6 inches", "6 stones", "6 kilograms", etc.

п. (3) и п. (4) - feet/foot является особым случаем, причём преимущественно в конструкции "5-foot-6", "six foot two" и т. п.
Говорить и писать "5 feet 6 inches" и "5 feet 6" столь же грамматически верно, как и "5 foot 6".

P.S. В русском интернете ни одного вхождения "5 фут 6" (с кавычками) Гуглом не найдено, а "5 футов 6" - сколько угодно.

VIadimir, спасибо за ссылку.
Что и требовалось доказать (с). :)
Использование формы единственного числа - foot - во всех прочих случаях, кроме двух упомянутых ситуаций, является допустимым в разговорной речи исключением, а не правилом.

Usage Note:
In Standard English, foot and feet have their own rules when they are used in combination with numbers to form expressions for units of measure: a four-foot plank, but not a four feet plank; also correct is a plank four feet long (or, less frequently, four foot long). When foot is combined with numbers greater than one to refer to simple distance, however, only the plural feet is used: a ledge 20 feet (not foot) away. At that speed, a car moves 88 feet (not foot) in a second.

Our Living Language Some people in New England and the South use constructions such as three foot and five mile in place of Standard English three feet and five miles in certain contexts. Some speakers extend this practice to measures of time, as in He was gone three year, though this is not as common. Interestingly, such constructions are used only if a specific numeral (other than one) precedes the noun. Thus, She gave me four gallon of cider can be heard in vernacular speech; however, no one would say She gave me gallon of cider for She gave me gallons of cider. This is because the numeral makes apparent the plural meaning that would not be specified if both the numeral and the plural form were omitted.

"I am almost fifteen stones."
Да?
или
"I am almost fifteen stone."
???