only u kn decide f da acknowledgment shud b short - i knt hlp u write it bcoz i don't no wut they did 2 help u.
Okay, some people will say that what I just wrote was not polite, but the point is, if you come to this forum asking for help in writing something, then please use your best attempt at correct English to ask for your help.
Are you speaking? Writing a hand-written thank you note? Writing the preface of a book that's about to be published? The medium, the audience, and, frankly, the actual depth of your appreciation all play a part in how long it will be and what you should say. If it should be longer, then say HOW their influence helped you. "Without her help in understanding the theory of relativity, I never would have learned the foundations that formed the basis of my award-winning science project, where I met my lovely wife Christine."