Do I need 42mm or 43mm?
That could depend on your guitar or your preference. Many prefer a 43mm nut over 42 regardless, because 43mm sometimes gives slightly wider string spacing, and/or the opportunity to sand the ends of the nut to precise width of the neck…or not. For an exact replacement, it’s best to measure your present nut or go with what is spec’d by your guitar manufacturer (see Fender website for example). Many Fender, Squire, and other Fender style guitars specify 42mm nut. Some spec 43mm or (42.8). The difference between 42mm and 43mm is very slight (42mm is approximately 1.65” or 1 5/8”— 43mm is approximately 1.69” or 1 11/16”). Important to keep in mind too, that sometimes manufacture specifications can differ slightly from what is actually on the guitar.