<apex:repeat value="{!newAttachments}" var="newAtt">
                    <apex:outputLabel value="Upload file"/>
                    <apex:inputFile id="inputfile" value="{!newAtt.body}" filename="{!newAtt.name}"  onchange="checkFileSize(event)" />
                </apex:repeat>

<script>
        function checkFileSize(event){
            if ($("[id$=inputfile]")[0].files.length > 0) {
                //validate file attachment is not > 500k
                var uploadFileSize = $("[id$=inputfile]")[0].files[0].size;
                if (uploadFileSize > 1024 * 1024 * .5) {
                    alert('File uploads must be less than 500k in size. remove files over: '+(Math.round(uploadFileSize/1024)) + 'k');
                    event.preventDefault();
                }
                else {
                    //
                }
            }
            else {
                // no file attached, if not required, call save method
            }
        }
        </script>