You should decide on your speakers first. Then worry about the receiver.
And, as far as "how much wattage does the most big tower speaker need", well, the answer is "more than it's maxium power rating". This was, I believe, touched on above, no? You might get a more useful answer if you ask a more useful question, like exactly what speaker are you talking about. Work with us a little.
Once you've decided on you r speakers, make a list of your toys outputs.
Now, look for receivers that have at least those inputs. The more, the merrier. This allows for future expansion.
Once you've got the field narrowed down a little, the rest falls into place.