Shave Handle and knot

Question?
If I buy a 24mm handle do I also buy a 24mm knot or a 23mm knot one size lower ?

You generally  want a knot with a diameter about 1mm smaller than the diameter of the handle socket at the top.

Thank you so much !

That's my understanding on knot diameter.  There is a lot of brush expertise on this forum, and perhaps others will post their views on your question.

I am no expert but I have done a few brushes. The knot depends on the manufacturer. Some advertise thier knots for the socket size. Others advertise knot size by the plug size. Some vendors state this in thier description. 

An example is Envy White. His 24mm knot requires 26mm hole. If in doubt, contact the vendor before purchasing.

In most cases, I have found that the knot size should be 2 mm less than the hole size.
I usually measure the actual knot 10 to 15 mm above the base in order to determine the hole size.
Many handles can easily be increased in size using a Dremel sanding drum.

Good luck and enjoy the process. I find it to be quite satisfying.