I'm pretty sure the stock fog light size is 3.5" (Haven't measure it though). That's the size of the kit that zkdreads bought and when I installed 3" fogs on our IOM LS there was about an extra .25" of space on both sides (so .5" total).
What you can do is buy 3" fogs that are projectors and have Halos likes
These and then instead of dremeling the whole middle part of the fog light covers you can dremel off only the inside part (the part that has ridges) and they'll fit in there perfectly.
That's how I should've originally installed mine, but it wasn't until the second fog light cover that I realized I shouldn't have dremeled the entire middle and by then it was too late. Here's pictures of my set up so you can get an idea.
Just Halos on:

Just Fogs on (not aimed):

View of them on the car:

Here you can see the little gaps on the side because I dremeled the entire middle:
Just a heads up incase you get an E-bay kit you may have to upgrade the wires that come out of the switch that's included in the kit. Here's a picture of the switch:
The wires that come out of that switch are extremely small and after a couple of extended uses of the fogs they all melted and my fogs stopped working. I ended up just replacing those really small wires with some wire I bought from Autozone and they've worked perfect ever since. I also ran my power source to this constant Voltage post on the battery terminal so I didn't have to hack into my turn signal wiring and then I ran the wire for the switch all the way up my drivers fender and into the cabin through the door so I didn't have to drill into the fire wall or find an existing hole (was being lazy lol).
P.S: Ignore the dates on those pictures, my camera resets the date every time I take out the batteries and I'm usually to lazy to reset it.
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2011 IOM LS - Drag DR 19s 18x7.5, Projector/Halo Fogs, Blacked out Gills, Red Calipers, Factory Spoiler, 15% Tint
2011 CGM LS- Injen CAI, Vmax Ported TB, Vmax Black Ice O'Lator, Carbon Fiber Bowties, Carbon Fiber Gills, Factory Spoiler, Stock SS Exhaust w/ Magnaflow x-pipe welded in & stock muffler delete, Miro 112 18x8.5 +35