I would still recommend a for loop and a separate variable which represents the largest distance. It will spare you a lot of work and won’t be any worse performance wise while you are also variable in the amount of actors if you wish 
First of all you need to trigger your get all actors of class. Since you want to check this all the time just hook it up to an event tick which is called each frame. Right here you should also reset your largest distance variable.
Behind it do a “ForEach” loop. in the body of that one do another “ForEach” with the same array. Compare the two “Array element” locations and if it’s larger than the largest distance variable set this as new largest distance.