This is going to be somewhat dependent:
- on the parts of the world you are targeting
- what parts of a name you need to include (middle names/initials?)
- any other abbreviations that might be needed (Mrs, Dr, PhD, MSc?)
- characters used in names, unless of course you're using a monospaced font
I don't think there's any substitute for sample test data from your intended markets, from which you should indeed be able to derive a 90th percentile, although failing to fit 1 in 10 seems problematic; I'd be tempted to go for 95th percentile or higher.
I happen to have a long list of first and last names here. Difficult to sort them all by actual length in mm, but if I sort them by number of characters, including spaces, the 95th percentile is 19 characters long, including a space.
I can then set all of my 19-character names in my chosen font size, visually check which is the longest and measure it in pixels or mm.
But that's just an example with mostly British data, and I didn't include middle names or initials, or titles; you should definitely do abit of research.