How big of an eye round........
I have always heard that you should allow for a minimum eight ounces per person for the main entree when preparing for a party. in recent years, I have heard the number go down to six ounces...though personally I would not agree with this trend. It is also suggested you never round down, but rather round up. Nothing could be more embarrassing than not having enough food for all your guests....and plus, there is no greater compliment to the host than when someone asks for second helpings...
You must also address the issue of shrinkage depending on how you will roast the beef. High heat will most certainly reduce the weight by a half pound. Slow roast, low heat will be less shrinkage and the preparation I would suggest to you. Believe it or not, the quality of your knives and knife skills in general also have a bearing in serving size. Unless you can make precise cuts of 1/4 or 1/2. the number of slices per serving comes into play. Obviously, the larger the slices, the more meat you will need to provide for your guests
So with due respect to (todao). I would have to disagree with the five pound roast and I would suggest a 7-9 pound or larger roast....especially if you want leftovers.
Here's a reference for you to consider: