Javascriptはtextareaの長さを制限します.

<script> 
function limitLength(obj, length) { 
    var desc = obj.value;  
    obj.value = substr(obj.value,length); 
} 
function substr(str,length){ 
    var l=0,i=0; 
    while(l <length && i<str.length){ 
        l+=1; 
        if(str.substring(i,i+1).match(/[\u4e00-\u9fa5]/))l+=2;//        3    
        i+=1; 
    } 
    return str.substring(0,i); 
} 
</script> 

  <textarea name="blogdesc" cols="50" rows="5" onkeyup="limitLength(this,200)">              </textarea>