LEN  

Returns the length of a given string

Use this  to return the length of a given  string. Needs to be in the format as follows.

LEN(input)

So here are some examples

=LEN("BLUE GREEN RED") which gives the answer 14

This would put the following formula in the input field once the dialog was dismissed via OK

=STR(LEN("BLUE GREEN RED"))

Any text containing spaces needs to be contained within "".