I love my Dremel, but sometimes I wish it was smaller. I started looking around for a cheap, small rotary tool and found an oldDremel 7700-1/15 MultiPro Cordless at a garage sale for $8.00. Unfortunately, after about a month or two of use it the battery started to die. Rather than buy a new battery pack for it I decided to seize the opportunity and make an even smaller Dremel. At just over 4 inches it is about 2/3 of the length and the same width of the smallest rotary tool I could find online, the Dremel 754-04 4.8V MiniMite, and it provides 1.5 times more power.
Disclaimer: this could potentially destroy your rotary tool. I have only done this on the Dremel MultiPro Cordless, I don't know what will happen with other brands or tools.