What is the standard size of the king?

There are so many different sizes of chess sets on the market nowadays but what is the standard size?

I personally would say 3.75 inches / 9.5 cm. Even though there is not an official standard size but tournament size tend to be an acceptable standard size. Since most tournament sets are at that 3.75 inches / 9.5 cm size. I would consider 3.75 inches / 9.5 cm to be the standard size. Standard size squares size for boards would be 2.25 inches / 57 mm. Go by tournament standards and this is the standard size for chess pieces and boards.  

The board size is not 57mm, i think its 55mm. 

I agree that the correct standard king height is 9.5 cm and the board size is 55 mm as these are the norms approved by Fide for chess tournaments. There are also bigger king sizes on the market but they mainly serve decorative purposes and are not accepted in formal chess tournaments. 

55 mm to 57 mm is pretty much standard. 2.16 inches to 2.25 inches that is why I went with 57 mm. I think that it is splitting hairs when we are talking about 90 thousandths of an inch difference. 

I thought that its because of the metrics used in USA. Because all the sellers in US use inches instead of mm or cm.

@Mohan_Kumar_Chess yes we in the USA use inches, I was just giving the metric to standard conversions of the size. I agree that 55 mm is quite standard as far as tournament boards go. So is 57 mm due to USCF standard of 2.25 inch squares. Either two are standard in my book. 

The difference between 55mm and 2.25" is 1/8th of an inch. DGT electronic boards used for many years in all major tournaments and WC matches have 55mm squares. Whether this is a noticeable difference is up to the user. Obviously either variation is fine for official tournament play. 

I think there is a flaw in your calculations. I used 1/8" difference as an approximation since that is easier to visualise for most people but it is not 1/32".

55mm = 2.165"

2.25" - 2.165" = .085"

That's 8.5% of an inch or 17/200 of an inch which is approximately 1/12th of an inch. Visually:

 Sorry for being a nerd! 😁 I think most people would be hard pressed to see this difference without a reference so either is fine imo.

As to whether the difference between 55mm and 2 1/4" is noticeable: https://www.chess.com/forum/view/chess-equipment/vintage-chavet-6-ebony-stain-d-yay

posts #16 to #19. Yes I am a total nerd with way too much time on my hands!

I was only off by 1/16 of an inch. It should have been 2 5/32" nearly equals 55 mm. 55 mm is in between 2 5/32" and 2 11/64". When there is only about 3/32" difference between 55 mm and 2.25". Not much of a difference. Whether 55 mm or 2.25" it's all good. 

Yep. Some people may even prefer the (marginal) extra space on 2 1/4" board. The Chavet set I was commenting on above is unique in the sense that although it was the WC/Grand Prix standard at the time, the king is only slightly above 3.6". With a normal 3.75" king it may not even be noticeable.

Well I don’t know about your king, but my king, well...