Edit in JSFiddle

//Plugin
(function($){
  $.fn.smsArea = function(options){

    var
    e = this,
    cutStrLength = 0,

    s = $.extend({

        cut: true,
        maxSmsNum: 3,
        interval: 400,

        counters: {
            message: $('#smsCount'),
            character: $('#smsLength')
        },

        lengths: {
            ascii: [160, 306, 459],
            unicode: [70, 134, 201]
        }
  }, options);


    e.keyup(function(){

        clearTimeout(this.timeout);
        this.timeout = setTimeout(function(){

            var
            smsType,
            smsLength = 0,
            smsCount = -1,
            charsLeft = 0,
            text = e.val(),
            isUnicode = false;

            for(var charPos = 0; charPos < text.length; charPos++){
                switch(text[charPos]){
                    case "\n": 
                    case "[":
                    case "]":
                    case "\\":
                    case "^":
                    case "{":
                    case "}":
                    case "|":
                    case "€":
                        smsLength += 2;
                    break;

                    default:
                        smsLength += 1;
                }

                //!isUnicode && text.charCodeAt(charPos) > 127 && text[charPos] != "€" && (isUnicode = true)
                if(text.charCodeAt(charPos) > 127 && text[charPos] != "€")
                isUnicode = true;
            }

            if(isUnicode)   smsType = s.lengths.unicode;
            else                smsType = s.lengths.ascii;

            for(var sCount = 0; sCount < s.maxSmsNum; sCount++){

                cutStrLength = smsType[sCount];
                if(smsLength <= smsType[sCount]){

                    smsCount = sCount + 1;
                    charsLeft = smsType[sCount] - smsLength;
                    break
                }
            }

            if(s.cut) e.val(text.substring(0, cutStrLength));
            smsCount == -1 && (smsCount = s.maxSmsNum, charsLeft = 0);

            s.counters.message.html(smsCount);
            s.counters.character.html(charsLeft);

        }, s.interval)
    }).keyup()
}}(jQuery));


//Start
$(function(){
    $('#smsText').smsArea({maxSmsNum:3});
})
<b id="smsCount"></b> SMS (<b id="smsLength"></b>) Characters left
<br />
 <textarea id="smsText" style="width:400px;height:200px"></textarea>