In order to import large XML files you will have to make sure that the following settings are set in your PHP.ini :

max_filesize_upload Default : 2M -> increase to 10M or larger

That is the size that is displayed close to the submit button on the import page.
In order to get it running you should also check your settings for memory_limit and max_execution_time. The first one is the setting for the amount of memory a script can handle and the latter one the maximum time for transactions. If the import takes too long it might get cancel’d because of that setting.