Troubles with a for loop

I am having trouble with the “Find the Longest Word in a String” challenge.
Basically - I have to find the longest word in the string and output the number of letters it has.
I am trying not to click the hint button as much as I can. I might be going in a completely wrong direction, but anyway…
This is what I imagined doing:
I wanted to split the given sentence into an array with words as values,and then I wanted to iterate through the array,take the iterated word and find its length,and at the end - use the sort function to sort it from the biggest to the lowest number.
After that - i would output the first (the biggest) value of the array. However - I am not understanding the way for loop works here… Check out the code I have written so far…

function findLongestWord(str) {
  var splitted = str.split(" ");
  for (var i = 0; i <= splitted.length; i++) {
    
  return splitted[i].length;
  }   
  return splitted;
}


The problem with the for loop I have is - I dont get all splitted[i].length that I expect. I only get my initialisation var i = 0;
returned. And what I wanted to happen is just have all my lengths returned in an array. This may not be the right way to solve this challenge,but I am very interested in how I could solve this problem I am having - I would love to know how to get all my .lenght values in an array , if that is possible...