Hey nobugging !
Super easy, here is a first solution, you need to select first all the objects you want to paste your value on, and then the last selection needs to be the reference object from where you want to copy the values, then click the first scale value in the Channel Editor and press Tab to copy the value to the all selection, as below ;
If you want an other scripted solution here is a short script you can run which does exactly the same ;
ref = "REFERENCE_NODE"
sel = cmds.ls(sl=True)
for obj in sel:
for axis in 'xyz': cmds.setAttr('%s.s' % obj, *cmds.getAttr('%s.s' % ref)[0], type='double3')
But honestly the first one is the fastest one, just need to get used with it 
This trick has been originally posted on 3dbunk, feel free to check out our others tips, tricks and tutorials !
Peace,
Mehdi
