Question:
1. What is the standard size of each squares in International Draughts?
2. Standard size of each pieces (diameter and thickness)?

@LuffyKudo
FMJD doesn't restrict game by standardizing size of boards and squares: they may be different within each federation or association, and some of them even allow to have free choices for any club. Same goes for colouring of board squares and thickness of pieces.
However, there are some unwritten rules which should be taken into consideration, and you will never get wrong if you observe them:
(a) In any official tournament, the diameter of any draughtsman should not be less than 35 mm and not more than 45 mm. In junior championships there might be exception, but I don't know any junior tournament (including blitz tournaments) where the diameter of draughtsman was less than 32 mm. Anything below 32 mm is regarded as amateurish and acceptable only for casual games.
(b) As for the board size, you should add some space (5 mm to 7 mm) to the board square, so that draughtsman can be arranged instantly.
Thus, if you have a set of draughtsmen which have 35 mm, the size of board square should be 40 mm to 42 mm. On 10x10 board we have 10 squares in any row and in any column, so the gaming area should be from 400 mm x 400 mm to 420 mm x 420 mm.
Besides, a good practice is to add some empty margin to gaming area: 5 mm to 7 mm from each side (left, right, top, bottom).
As for colouring, keep in mind that, contrary to popular beliefs, the draughts board is not painted in black and white, and the most conservative club players refuse to play on black and white boards, because the visibility is very low on them! The dark square colouring varies from one country to other country, and from one region to other region. The most popular colours for dark squares are green, brown and red in many variations (for example, olive green).
(c) As for material for draughtsmen, many people might assume that wooden pieces are the most suitable; however, for practical purposes, plastic draughtsmen are the best. Take into consideration that wooden pieces tend to broke and they are less suitable for stacking, but you should stack two draughtsman as a sign that you have a king!
Similarly, because stacking is needed, the thickness of draughtsman should be appropriate: not less than 5 mm and not above 10 mm.
In a recent post I said that many draughts players use backgammon pieces. It is because backgammon players share same and similar requirement of piece thickness! Therefore, you can consider playing with draughtsmen which were originally designed for playing backgammon.
Technically, for playing blitz games, you would like to play with draughtsmen that have a small hole in the central part, so you can move a draughtsman quickly without extra tension to hand. For illustration purposes, see examples in www.playtables.lv/uploads/catalog/px548/photo_9443.jpg and katgammon.com/wp-content/uploads/2022/05/Checkers-001-600x400.jpg .

2

I see, thank you so much @kalnap, God Bless =>