The reason I went this way with a battery backup is because I could use it with any case I may have on my phone...
It is 3 1/2 inches long and 2 wide is all... and just under a 1/2 thick... for the model below that is on ebay...
http://cgi.ebay.com/3G-iPHONE-iPOD-i...6.c0.m14.l1318
This one is a little larger in size but not bigger than the phone itself...
http://cgi.ebay.com/EXTERNAL-BATTERY...6.c0.m14.l1318
If you go to the links that are on ebay you will be able to view pictures of these two...