how to call function when width is below 600px using jQuery and call it again when windows size changes?

You can use $(window).width() to check the current width of the screen using jQuery and fire function's which need

if($(window).width() < 600) //don't need to write px in the end , only numbers
{
   //call a function
}

Suppose you need to call a function CheckWidth() on page load and whenever window size changes, you have to write code like this

function CheckWidth(){

 //do something if window size is belo 600
 if($(window).width() < 600) 
 {
   //code
 }
 else
 {
   //else do this
 }
}

//call function on page load
CheckWidth();

//call function whenever windows size changes
$(window).on('resize',function(){

   CheckWidth();
})

That's it, you are done.