I don't consider myself having a large head all other headgear I have is medium. This skull cap is marked S/M but it's definitely small. It's a very tight fit, too tight in fact.
A very well made product fits and covers well under helmet ,warm and kept the rain out also looks good when worn without helmet
L/XL fitted my 59cm head diameter. Not too tight or slack. Covers ears nicely and helmet fits snugly over it. Stops a cold head down to about 5° air temp. Half price bargain at the time of purchase, so v happy.
It does what I expect it to do, keep my head and ears warm.
Good fit for my 59cm head - felt a bit tight on first use but has eased over the next 2/3 rides and now very comfortable. Works well down to 0 Celsius - haven't tried it any colder. Washes well. Can't really comment on durability other than it looks to be well made. Significant improvement on other (improvised) cold weather solutions e.g. old woolly hat. Very happy to recommend.