How do I know what size my HDD is?

I'm looking for an external enclosure for one of my HDDs from a desktop computer. How do I know what size it is (i.e 3.5 or 5.25 or whatever)?

I seriously doubt it's going to be a 5.25" drive. If it's from a desktop, it's probably going to be a 3.5". If it's from a laptop, it'll be a 2.5".

That's what I figured. But how does one measure it? What does the measurement mean? None of the dimensions are either 3.5 or 5.25.

if you've got a pc around simply look at the size, the hdd in the pc will be 3.5 and if its smaller its 2.5

I beleive it refers to the size of the platters inside. So basically the width of the drive. If you post a picture of it mostly showing the back, I could tell you what type it is.