There is no upper limit. Often used in an aspirational context - you can do anything you want.
Sources differ on this one.
The Random House Dictionary of American Proverbs and Sayings edited by Gregory Titelman is of the opinion that Cervantes wrote "no limits but the sky" in his 1605 work Don Quixote.
But the usually reliable phrases.org.uk is convinced that Cervantes did not say this in the original Spanish and that the expression was introduced in early English translations of Don Quixote. However, confusingly, it then goes on to say that the earliest citation in English is from the Syracuse Herald (New York state) in September 1911, and that the phrase is influenced by the invention of the airplane.
With little grasp of Spanish, and no grasp at all of early 17th century Spanish, BookBrowse has no opinion on whether the phrase does appear in Cervantes' original, but we did find it in an 1880 English translation at Google Books - in a copy of The History of Don Quixote of La Mancha translated from the Spanish by Motteux edited with notes and memoir by John G. Lockhart, published by J. C. Nimmo and Bain in London.
So, the jury is out. It seems certain that the expression predates the 20th century due to its inclusion in the aforementioned 1880 translation of Don Quixote; but whether the expression was coined by Cervantes or by an inspired early translator will have to be left to greater minds than ours to resolve, particularly considering that in 1989 a first edition of Don Quixote sold for $1.5 million, so we won't be getting our hands on one of those anytime soon!
