Cordless drill features: Chuck size
A chuck is a specialized type of clamp used to hold materials or rotating tools.
A drill chuck is a specialised three-jaw chuck, used to hold drill bits or other rotary tools. 
The image above show the traditional keyed style of drill chuck. These chucks require a key to provide the necessary torque to tighten and loosen the jaws. The rotary action of the key turns the outer body which acts on an internal screw; this in turn moves the threaded jaws in or out along a tapered surface. The taper allows the jaws to encompass various sizes of drill shanks.
Chuck size is important feature in determining the purchase of a cordless drill. At cordless drill market you can find two main sizes, cordless power drill with a 1/2" chuck and cordless power drill with a 3/8" chuck. With a 3/8" chuck size you are limited to only using drill bits with a 3/8" shank. Anything larger needed for your project you would need a 1/2" chuck size. But with the 1/2" chuck you can use all bits with a 1/2" shank, and most bits that are 5/8" or larger.
For the average homeowner for a general-purpose tool a 3/8" should be fine, however for heavy-duty work a half-inch (1/2") chuck drill should be the best choice.
Also I suggest to choose keyed drill chuck since it is very hard to loosen up jammed drill bit with keyless drill chuck.
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