The Rancilio parts list (first result on http://bit.ly/gTKKvb) shows it as a M5x14 (stainless steel) although I think older Silvias may have been M5x12 (eg. http://www.coffeeparts.com/domestic-parts/machines/rancilio-silvia-09.html).
Theres two ways - the easy but not so effective way is to just use a button head socket screw (eg. http://boltsnutsscrewsonline.com/316-ISO7380-BHSS-M5-X-12.3694) or the M5x16 and shorten it a bit (what I did).
The harder way is to get a countersunk screw, modify the Water Distributor to suit and then bend the shower screen between them - there are better instructions around here somewhere - try search.