The chaff is 1/2 the wavelength of the radar signal. The idea is that if the antenna (or chaff strip in this case) is some multiple of half the wavelength of the signal then it resonates, just like a guitar string. And since you generally want the use the least amount of material possible, 1/2 wavelength is pretty much the way to go. If you're interested in how all of this works, get the ARRL Antenna Handbook, it's both chock full of details and reasonably easy to understand explanations.
For chaff, the idea is to return as much of the signal as possible to produce the largest effect on the display as you can. The resonance allows that signal to build up over the length of the pulse, whereas using something farther away from the resonant length would mean it faded away more rapidly and the total energy at the end would be lower. I seem to recall the early UK Window was backed with paper, so it was likely slightly longer than 1/2 wavelength due to dielectric effects, but I'll have to look that up.
Historical note: the original calculation for radar made by Wilkins in 1935 relied on this effect. He noted that a contemporary bomber had a wingspan about 25 metres (for instance, 23m for the He 111), which made it just about a perfect half-wave dipole for signals of 50 metres. 50 metres just happened to be the frequency the BBC had recently selected for a new transmitter, so they demonstrated radar by using that transmitter, their own receiver, and flying an aircraft around the area.
And another: the Germans also developed chaff and tested it near Denmark on occasion. They were too scared to use it though, because they thought the first time the did the RAF would figure it out and return the favor. Since the RAF had a more powerful bomber fleet, that would be bad. So they sat on the idea, and burned the documentation to prevent it leaking. Ironically, about a year later RAF had a HUGE argument over whether to use window on Germany. You see, they were afraid that the first time the used it that the Germans would figure it out and return the favor... In this case, however, the introduction of a new radar set working on a frequency the Germans didn't know about eventually led to it being release for use with spectacular results. The Germans did return the favor in 1944, but the new radars saw right through it. Thank you Randall and Boot.