[SalesForce] Size of an attachment

I really don't know how catch the uploaded file error using apex (server-side), but I've avoided that limitation using javascript (client-side) before the form is submitted in order to notify the user about the limitation.

This is an example:

    <apex:page standardController="document" >

        <apex:form styleClass="js-wrapp">
            <apex:inputFile value="{!document.body}" filename="{!document.name}"/ >
            <apex:commandButton value="Save" action="{!save}"/>
        </apex:form>


       <apex:includeScript value="https://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"  />
       <script>

function getIEFileSize(file){
     var myFSO = new ActiveXObject("Scripting.FileSystemObject"),
    filepath = file.value,
    thefile = myFSO.getFile(filepath);
    return thefile.size;
}

   $(function(){
    $('form.js-wrapper').submit(function(){
        var isOk = true;
        $('input[type=file]').each(function(){
            if(typeof this.files[0] !== 'undefined'){
                var file = this.files[0],
                size = typeof ActiveXObject !== 'undefined' ?
                    getIEFileSize(file)
                    :
                    file.fileSize || file.size;

                isOk = 5242880 > size;
                if(!isOk){
                  alert(this.files[0].name +' file is bigger than 5MB');
                }

                return isOk;
            }
        });
        return isOk;
    });
   });

       </script>

</apex:page>

The script will work for any amount of file inputs checking all of them.

With Summer '14 release, file size limits for attachments were increased to 25MB, so you should be fine:

http://docs.releasenotes.salesforce.com/en-gb/summer14/release-notes/rn_general_attachment_size_increase.htm