I'm looking for a smaller battery for my 2007 S40 so I can make room for a round oil tank under the seat.  Does anyone have suggestions for a good battery that's significantly smaller than stock?  I'd like to stay under $100 if possible.  

Thanks!

I bought the same EarthX battery ast GyroBob....except I bumped up to teh 12A battery.  Same size case but twice the amp hours.

http://earthxmotorsports.com/products-page/cbr1000/etx12a/

After having ridden with it for a couple of months the 6A would probably work just as well.  I don't ride when it is cold, my bike is stored indoors, and when I hit my starter button my bike never turns over more than a second before the bike is running.  I njever have to cank for extended periods.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1259644714

Above is the link to the write up on the mini battery install. I used the battery in the write up ans its Way smaller than stock and I give it 2 thumbs up.

I too was running a EarthX battery 12A and though the 135A CCA that the 12A produces are way to low for the Savage it did the job. However since I put the 97mm high compression Piston in it struggled to turn over and then failed completely. To get the CCA required I would have needed to go to a ETX24C or even bigger. Which also meant bigger case.
So I went for a Aliant X3 Batt which is slightly bigger in size but with 300A CCA. And it still fits in my converted air box.
http://www.bmaglobal.net/aliant-batteries