Its one object like in my attached capx
I try fix this when i return home
Are all the sprites one object or iseparate? If it is one object you can use a loop and a counter.
First set CounterVariable to 0.
Repeat Sprite.Count times - Add Sprite(loopindex).Width to CounterVariable.
Make sure the loop runs by trigger and not every tick.
If they are separate objects you should be able to do the same thing with family.