Add up the width of each column in the index, then add the width of the clustering key, and multiply the whole lot by the number of rows in the table (assuming we're not talking about a filtered index). That'll give you an approximate figure, but bear in mind that it can be influenced by fill factors, non-leaf pages (if you have a lot of of rows in your table) and the number of rows that can fit on one data page (if your index is particularly wide).
John