Anything over a 3ft. slider becomes cumbersome for a small crew, and longer sliders start requiring additional stands to set up. I prefer working with sliders around 3' max. Take a look at this 28" 360VM slider with belt and crank pulley. Yup, similar to that of the V1 Kessler Pocket Dolly which used to sell for upwards of $600+ dollars. A similar CamTree/ProAim version with crank pulley still runs well over $400+. Your most basic IGUS DIY rail at this width could run you about $200+ without the crank setup. That's what makes the price on this 360VM slider pretty competitive with the current offerings. There's absolutely no reason to go with anything much larger, especially for those migrating to smaller camera setups like the new Sony NEX-5n, NEX-7, Olympus EP-3, etc.
Here's a video below that shows the actual 360VM slider in use, and is seen in the video.
Putting a Crank and Pulley on one of IGUS' widest rails was mostly known through Kessler's Pocket Dolly V1.
Some of those videos found here: http://vimeo.com/14222897
Another Example here: http://vimeo.com/14848131
The installed pulley makes it even sweeter for those looking to set up a motorized motion controlled video dolly. A simple motor and reversible PWM speed controller, and you're good to go. The 28" Crank slider can be found on the 360VM.com website here: http://store.360vm.com/product/philly-slider, but there may also be a longer version found here: http://store.360vm.com/product/360-video-camera-slider-38-inch