How to find HDD cache size?

I Googled my HDD model number, and could only find the physical dimensions and the data transfer rate. Where can I find the HDD cache/buffer size?

There's a software called CrystalDiskInfo which can be downloaded from here. This software is free and lists out details such as Firmware, Buffer size (what you need), Interface, Serial number, and also S.M.A.R.T details.

Actually your true transfer rate will depends on the system that you are running it. For example, if your USB controler isn't too fast, your external hard disk will not reach its nominal transfer rate.

I search for my HDD buffer size with Windows msinfo32 (type "msinfo32" on a cmd prompt) and on SIW software but I didn't find this specific information, However, I hope all of these specifications you will get on manufacturer datasheets, for Western Digital, you will find here (just search for your model part number). If you want to test the trully transfer rate of your external device, I would recommend you using HD Tune or HD Tach. Both are benchmark programs and will shows you detailed information about your device capabilities(SMART) and real rates.

Most, if not all, drives will have it written on their label. So if you can physically take a look at the drive, try that. If the drive is hard to reach and you have the model number you could do a Google Image search and probably find a picture of the label.