//判断照片大小
function getPhotoSize(obj){
    var fileSize = 0;
    var isIE = /msie/i.test(navigator.userAgent) && !window.opera;            
    if (isIE && !obj.files) {          
         var filePath = obj.value;            
         var fileSystem = new ActiveXObject("Scripting.FileSystemObject");   
         var file = fileSystem.GetFile (filePath);               
         fileSize = file.Size;         
    }else {  
         fileSize = obj.files[0].size;     
    } 
    fileSize=Math.ceil(fileSize/1024); //单位为KB
    
    return fileSize;
}
<input name="file" size="30" type="file" onchange="getFileSize(this.value)" />