how to use string.len on only one word of a string ?

so im working on a admin script and i need to be able to do ;/rank PLAYER (NUMBER) i made the rank command and tried lots of things checked the wiki and i cant find it you see i need to be able to check the length of the players name and add 2 to that then that is where you input the number how would i go about doing this ive tried everything i can think of