Responsive text by JavaScript
Thoughts:


Variable names: I hate typing unnecessary text as much as the next dev, but variable names are not a good place to skimp. It's one thing when using i as an index variable, for example, but for others, please, please, please use meaningful names. If you have to provide a key after the fact, this should be a good clue that you've not adequately done so.
Don't pollute the global space. If the caller needs access to the function, return it instead. Or use modules (module.exports or the like).
minFontSize less than zero doesn't make much sense. I'd cap it at zero rather than –∞.
maxFontSize being capped at +∞ probably doesn't make sense either.
Your code will re-parse the minimum and maximum font sizes each time the handler is called. Cache these instead.
var a = k: Why? In this case, just use k instead.
i() does not do what you think it will: if el is an array, el.style is going to be undefined, and el.style.fontSize will error.
When iterating arrays using for (;;;), cache the length, like so:

for (let i=0, l=arr.length;i&lt;l;i++) { ... } // for ES5 use "var"

If k = 0, what happens? Division by zero...


This is a very quick rendition of your code (in ES6), with meaningful variable names, and implementing caching and such, but it might be useful:

function fitText(el, ratio, {minFontSize, maxFontSize}) {
  el.style.fontSize = Math.max(Math.min(el.clientWidth / ratio, maxFontSize), minFontSize) + "px";
}

function attachListeners(el, ratio, {minFontSize, maxFontSize} = {}) {
  const boundFitText = fitText.bind(undefined, el, ratio, {minFontSize, maxFontSize}),
        addEventListener = window.addEventListener;
  addEventListener("resize", boundFitText, false);
  addEventListener("orientationchange", boundFitText, false);
}

export function adjustTextSize(el = [], ratio = 1, 
  { 
    minFontSize=0, 
    maxFontSize=Number.POSITIVE_INFINITY 
  } = {}) 
{
  if (ratio === 0) {
    return el; // prevent division by zero later; throwing an exception might be better here, though
  }

  const parsedOptions = {
    minFontSize: parseFloat(minFontSize),
    maxFontSize: parseFloat(maxFontSize)
  }

  const els = (el instanceof Array) ? el : [el];
  for (let i=0, l=els.length; i&lt;l; i++) {
    let actualEl = els[i];
    fitText(actualEl, ratio, parsedOptions);
    attachListeners(actualEl, ratio, parsedOptions);
  }
  return el;
};