Getting size of an array passed in as an argument

Of course, you can do it – it’s just len is not a field, it’s a method:

fn test_length(arr: &[String]){
    if arr.len() >= 10 {
        // Do stuff
    }
}

If you only started learning Rust, you should read through the official book – you will also find why &[str] does not make sense (in short, str is unsized type, and you can’t make an array of it; instead &str should be used for borrowed strings and String for owned strings; most likely you have a Vec<String> somewhere, and you can easily get &[String] out of it).

I would also add that it is not clear if you want to pass a string or an array of strings into the function. If it is a string, then you should write

fn test_length(arr: &str) {
    if arr.len() >= 10 {
        // Do stuff
    }
}

len() on a string, however, returns the length in bytes which may be not what you need (length in bytes != length in “characters” in general, whatever definition of “character” you use, because strings are in UTF-8 in Rust, and UTF-8 is a variable width encoding).

Note that I also changed testLength to test_length because snake_case is the accepted convention for Rust programs.