Blender: Learning how to create a disc

I've started teaching myself how to create a 3D object. Last time, I learned how to create a sphere (SHIFT-A > Mesh > UVSphere) in Blender, an open-source 3D graphics programme.

Through a combination of accidental button-mashing, I've learned how to rotate the camera slightly: using the scroll-wheel and different combinations of ALT, CTRL, CTRL-SHIFT.

So, 1.8 goals down. Now to try and figure out how to make a disc and then make it 39mm in diameter and 3mm thick.

Checking the Mesh > Shape options, it looks like 'cylinder' gives me exactly what I want. But the default cylinder is too thick. I want to adjust its dimensions.

There has to be something on-screen that'll let me define that. Looking at the Object tools menu, there's a section that's specifically about the cylinder--and it has categories for vertices, radius, and depth.

Let's see what adjusting the depth does ...

So it looks like adjusting the dimensions is easy. But how do the units that Blender uses correspond to the real world? What is a radius of 1 in millimeters?

Googling 'blender measure' brings up a couple of tutorials and Q&As:

http://www.katsbits.com/tutorials/blender/metric-imperial-units.php

https://blender.stackexchange.com/questions/254/how-do-blender-units-and-meters-feet-or-yards-correlate

It looks like there's something called the 'Scene' tab, which has a 'Units' option in it. And in Units, there should be options for:

• None
• Imperial
• Metric

... but after a thorough look around, I don't see 'Units' in the Scene tab. ... And my attempts to add one lead to frustration and losing all the default toolbars. After restarting Blender I find the Units section they're talking about ... but only by observing which button is highlighted blue at the top of this picture in one of the tutorials):

A small bit of fiddling around later, and I've created a 39x3mm token!

(It's pretty small in this picture.)