$(function() {
  $('select').on('change', function() {
    //Lets target the parent element, instead of P. P will inherit it's font size (css)
    var targets = $('#content'),
      property = this.dataset.property;
    targets.css(property, this.value);
    sameheight('#content p');
  }).prop('selectedIndex', 0);
});
var btn = document.getElementById('go'),
  textarea = document.getElementById('textarea1'),
  content = document.getElementById('content'),
  chunkSize = 100;
btn.addEventListener('click', initialDistribute);
content.addEventListener('keyup', handleKey);
content.addEventListener('paste', handlePaste);

function initialDistribute() {
  var text = textarea.value;
  while (content.hasChildNodes()) {
    content.removeChild(content.lastChild);
  }
  rearrange(text);
}

function rearrange(text) {
  var chunks = splitText(text, false);
  chunks.forEach(function(str, idx) {
    para = document.createElement('P');
    para.setAttribute('contenteditable', true);
    para.textContent = str;
    content.appendChild(para);
  });
  sameheight('#content p');
}

function handleKey(e) {
  var para = e.target,
    position,
    key, fragment, overflow, remainingText;
  key = e.which || e.keyCode || 0;
  if (para.tagName != 'P') {
    return;
  }
  if (key != 13 && key != 8) {
    redistributeAuto(para);
    return;
  }
  position = window.getSelection().getRangeAt(0).startOffset;
  if (key == 13) {
    fragment = para.lastChild;
    overflow = fragment.textContent;
    fragment.parentNode.removeChild(fragment);
    remainingText = overflow + removeSiblings(para, false);
    rearrange(remainingText);
  }
  if (key == 8 && para.previousElementSibling && position == 0) {
    fragment = para.previousElementSibling;
    remainingText = removeSiblings(fragment, true);
    rearrange(remainingText);
  }
}

function handlePaste(e) {
  if (e.target.tagName != 'P') {
    return;
  }
  overflow = e.target.textContent + removeSiblings(fragment, true);
  rearrange(remainingText);
}

function redistributeAuto(para) {
  var text = para.textContent,
    fullText;
  if (text.length > chunkSize) {
    fullText = removeSiblings(para, true);
  }
  rearrange(fullText);
}

function removeSiblings(elem, includeCurrent) {
  var text = '',
    next;
  if (includeCurrent && !elem.previousElementSibling) {
    parent = elem.parentNode;
    text = parent.textContent;
    while (parent.hasChildNodes()) {
      parent.removeChild(parent.lastChild);
    }
  } else {
    elem = includeCurrent ? elem.previousElementSibling : elem;
    while (next = elem.nextSibling) {
      text += next.textContent;
      elem.parentNode.removeChild(next);
    }
  }
  return text;
}

function splitText(text, useRegex) {
  var chunks = [],
    i, textSize, boundary = 0;
  if (useRegex) {
    var regex = new RegExp('.{1,' + chunkSize + '}\\b', 'g');
    chunks = text.match(regex) || [];
  } else {
    for (i = 0, textSize = text.length; i < textSize; i = boundary) {
      boundary = i + chunkSize;
      if (boundary <= textSize && text.charAt(boundary) == ' ') {
        chunks.push(text.substring(i, boundary));
      } else {
        while (boundary <= textSize && text.charAt(boundary) != ' ') {
          boundary++;
        }
        chunks.push(text.substring(i, boundary));
      }
    }
  }
  return chunks;
}

 function sameheight(selector){
var max_y=0;
var y=0;
$(selector).css('height','');
$(selector).each(function(){
  y=$(this).outerHeight();
  if(y>max_y) max_y=y;
});
$(selector).css('height',max_y);
  }

#text_land {
  border: 1px solid #ccc;
  padding: 25px;
  margin-bottom: 30px;
}
textarea {
  width: 95%;
}
label {
  display: block;
  width: 50%;
  clear: both;
  margin: 0 0 .5em;
}
label select {
  width: 50%;
  float: right;
}
* {
  box-sizing: border-box;
  padding: 0;
  margin: 0;
}
body {
  font-family: monospace;
  font-size: 1em;
}
h3 {
  margin: 1.2em 0;
}
div {
  margin: 1.2em;
}
textarea {
  width: 100%;
}
button {
  padding: .5em;
}
p {
 /*Here the sliles for OTHER paragraphs*/
}
#content p {
  font-size:inherit;/*So it gets the font size set on the #content div*/
  padding: 1.2em .5em;
  margin: 1.4em 0;
  border: 1px dashed #aaa;
  overflow:hidden;
}

p {width:400px;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="styles">
  <label>Font-size:
    <select data-property="font-size">
      <option disabled>
        Select font-size:
      </option>
      <option>
        smaller
      </option>
      <option>
        10px
      </option>
      <option>
        12px
      </option>
      <option>
        14px
      </option>
      <option>
        16px
      </option>
      <option>
        18px
      </option>
      <option>
        20px
      </option>
      <option>
        larger
      </option>
    </select>
  </label>
</div>
<div>
  <h3>Paste text in the field below to divide text into paragraphs..</h3>
  <textarea id="textarea1" placeholder="Type text here, then press the button below." rows="5">
  </textarea>
  <br>
  <br>
  <button id="go">Divide Text into Paragraphs</button>
</div>
<div>
  <h3 align="right">Divided Text Will Appear Below:</h3>
  <hr>
  <div id="content"></div>
</div>