how reduce the size of all photos in my iPhoto library?
Before you take this (drastic) step, consider a couple of things. 


Make sure the trash in iPhoto has been emptied -- not everyone realises this needs to be done, and it can take gigs of space.
iPhoto creates a duplicate image when it rotates an image, and the original is not required. I'm not sure if there's an easy manual way to remove these, but I use CleanMyMac (http://macpaw.com/cleanmymac). It has a free trial and it will save a large amount of space. 
If you're worried about performance due to the size of the library, consider splitting it up helped me. I got to about 60gb, and it was very slow, so I used iPhoto Library Manager (http://www.fatcatsoftware.com/iplm/) to split it (I split into 1 library per year)