What would mining whole chunk mean?

What Kenjara said about chunk size is correct. However I would add the following points.

• Diamond Ore only appears below level 16
• The highest concentration of diamonds occurs between levels 5-12 but because of lava lakes there is slightly less in the middle of that (~9)
• There is ~1 diamond ore vein generated per chunk.
• An ore vein will have between 3 - 8 diamond ore in it.
• However, that vein can be overwritten by other generated structures - such structures, such as caves, can leave you with a chunk with no ore vein in it.
• Because of this, it is possible to mine an entire chunk and still not get any diamonds.

Because of these points and how long it would take to mine an entire's chunk worth of diamond possible blocks there are more efficient methods than mining every block in the range. If you want to be absolutely certain you don't miss it the most efficient method is lane mining. This is the method where you expose the maximum amount of blocks for inspection while mining the minimum amount.

So pretend this is a top down view of a layer of stone

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You would want to mine like this

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So you leave two lanes of unmined stone between each mined lane. This lets you see the contents of both of the unmined lanes by walking down the mined out lanes, and reduces the amount of total mining by to a third of what mining a whole chunk would take.

A single chunk is A 16 X 16 area 256 blocks deep. It is probably saying to mine a 16 x 16 area at the level diamonds appear.

As far as I am aware this will not certainly get you diamonds. You are probably better off looking for dirt at the levels diamond exist and mining all of the dirt as diamonds can appear among it.

For diamond mining tactics check out the wiki. http://minecraft.gamepedia.com/Tutorials/Diamonds