I ordered a western digital caviar blue 250gb 3.5 7200rpm internal hard drive, but when I received it, the sticker on the drive said 320GB in large letters. A receipt in the box said 250GB. I have seen flash drives and small data storage devices be advertised as 8GB, for example, and had the computer read it as something like 7.52 GB, so is this the "true size" of the hard drive, or was the wrong hard drive sent to me?

WD makes both 250 and 320 GB Caviar Blue hard drives. If the sticker and assume part number indicate a 320GB drive ( i.e. WD3200AAXX) than the wrong drive was sent to you. Could be many reasons...

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This is definitely some mistake they made. You have to check with online retailer.

Drive manufacturers publish the size in Gigabytes which is similar to the way you weigh other stuff. For example 1Kg = 1000gms similarly 1GB=1000 MB. So when they say 8GB they mean 8000MB. However, your computer or you can say OSes measure in terms of 2^ notation. They basically measure in Kibi, Mebi and Gebi bytes. Which means for system 1 GiB = 1024 MB.

That is the reason you see difference in size because disk manufacturers publish it in GB (Gigabyte) and system reads in GiB (Gibibyte).

You might want to have a look at: http://en.wikipedia.org/wiki/Gigabyte and http://en.wikipedia.org/wiki/Gibibyte for more details.

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Usually larger retailers will sent you a larger disk if they ran out of stock for the smaller disks. For them the price difference is negligible (especially with those small disk sizes) and the costs to call you back telling you that the part is out of stock and asking you if you want to wait, order something different or cancel the order are way higher than just giving you a larger disk.
But stuff like that is usually also mentioned on the receipt. The important thing for you is that the serial number on the disk is the same as the serial number on the receipt. If it is the same, lucky day for you, you just got a few GB as gift, if it is not the case you will have to contact the retailer because you will otherwise have serious troubles to RMA the disk in case of malfunction.