You want to buy a smoking cap, but it’s not “ones size fits all”.
So how do you choose a size? 

Measuring or even knowing your head size is not something we think about every day, 
and sizes may vary between brands. 

Here is a quick “how too” guide.  

To measure your head
You will need either a soft tape measure (dressmakers tape measure)
or a piece of string and a regular tape measure, to measure it against.  

Portrait of a Victorian male with a line showing where to measure your head

Measure the circumference around your head,
keeping the tape level and firm,
just above the ears, across the forehead and over the bump on the back of your head.