What is the maximum size hard drive that can be installed in an IDE MyBook 250gb external hard drive case.?

You should check this in your manual. Here I don't see any details of the device (is it Western Digital something or a shelf hanging on the wall and named "MyBook"?).

The maximum size specified by the manufacturer might not be the true maximum - there are multiple factors that affect this. One of them is the firmware of this device. Other might be some hardware component contacting through the IDE.

Note that the disk size is usually specified as "decimal" instead of "binary" units (320 000 MB is usually 320 000 000 000 B while computer would show 305MiB).

I doubt if anyone could tell you the value - you should rather check different drives and find this out by yourself.

Good luck

Edit

Chad gave you information which might be invalid. Western Digital MyBook products are US products (what I have checked before providing the answer) thus Chad's input, which seems to be based on the assumption that this is just an IDE-to-IDE, seems to be incorrect.

Bodyrot gave similar answer to Chad - I doubt if they have seen that this HDD case is connected using USB thus there is internal electronics that must act as an interface between your PC and the IDE drive. If anyone is going to say again that any IDE drive will work then please, explain to me why multiple motherboards that I have worked with cannot support ex. HDD larger than X GiB (ex. 32GiB for MVP3 chipset based mother board "PcChips M577" which can support up to 128GiB when BIOS patch is applied)...

At http://www.wdc.com/en/PRODUCTS/index.asp?Cat=8 you might find some details about the drives.

I doubt if you can find the maximum capacity - but it is likely that you should be able to install a drive that is about the size of maximum factory size for your series (ex. "Western Digital My Book Essential" comes with capacity between 0.5TB and 2TB thus it should support at least 2TB).

As I have already written - there are multiple factors that can affect the limit thus you should rather try with several drives (you might even try connecting them 3.5" drives if you can pull the cable enough - do this just for testing purpose).

Probably there is a size limit. Probably your limit might be one of the following: 32GiB, 64Gib, 128GiB, 320GiB, 512GiB, 2TiB.

Simple answer is that ANY size HDD that falls in the right dimensions will work. The only factor would be the computer you're using it with.

I'm fairly certain that there is no limit as to the amount of memory, as long as it has the right connections and is the right size (3.5" or 2.5" etc.)