Importing large RSS files in WordPress has its share of bugs and problems. A major cause is due to the latest configuration changes with PHP 5.2. Regular expressions are now capped at 100KB in PHP. WordPress uses the PHP function preg_match_all() to parse out the RSS entries.
If you import a post greater than 100KB, the import function to fail completely.
If you don’t have access to change the PHP server settings, simply edit the RSS file and remove the large RSS posts from the file.
If you have access to a server:
To import large RSS posts into WordPress, you will need to change the php.ini configuration on your server.
;PCRE library backtracking limit.
I’ve set it to 5MB, up from 100KB. PHP added this limit to prevent the server from running out of memory. Change it temporarily to 5000000 for the import process. Restart Apache/nginx/etc. to set the values.
Open the WordPress importer, select your RSS file from your computer, and run the import. Once you are able to import the RSS file posts, change the configuration back to:
Restart your web server to revert the changes.