Re: What Size Box for outside Wall Light Fixture???
#1 this is called an E block
#2 make sure there is an inch or so more of surface of the e block face than the mounting plate on the fixture itself
#3 i would avoid silicone, though alot of people swear by it, ive seen more caulking failures with silicone than poly butyl( flex), use a tube of flex or quad, whichever most siding suppliers sell this. set the box into a bead of this then tape off the nailing flange with either red tape or vycor.
now take into into note, if you have siding which requires a rain screen you will have to allow for this with the thickness of the rain screen stock, if using 3/4" stock you will have to nail the block to 1/2" plywood backing, by the time the block goes on and its taped up then the siding goes on, the siding will bulge out if you use 3/4" stock to mount the block to, going to thinner material will allow the siding to run flat
fire up the saw and make some dust