Width (across the fretboard) and depth (distance from fretboard edge to headstock edge) are the most important dimensions to get as close as possible to the kit nut dimensions. The kit nut heights are often too high, so you really need to measure the minimum height you’ll need off the guitar.
You need to get the width the same as or just over the neck width at the nut. This should be either 43mm or 42mm (I’ve had both on the kits but 43mm is more common). I’ve found you can sand down to the right width by up to 1mm without the strings sitting too close to the edge of the board, but you do need to take the same amount off both ends. What you don’t want is a nut where the ends don’t sit flush with the sides of the neck.
The depth can be a little greater as the headstock side can be sanded down to fit, and the height will always need to be adjusted, even if it’s for a big manufacturer’s standard guitar like a Strat, Tele or Les Paul.