said by deal1:
I suggest you install Tomato firmware and if QOS is one of the main features you want working well then Toastman's version of Tomato is a great choice. He has some good writeups on how to use it linked off his site.
...but to answer your question you would do a few speedtests and then insert a number 20% less than your average actual speed.
Pretty solid advice. I have been using Toastman for about a year with a VOIP setup. I have been using 15% less than my actual speed and it has given me no problems. YMMV.