I think both OE and Outlook run to 2gb.
Two short-term solutions:
1) Use a programme like Mailwasher to filter out unwanted stuff before you download
2) Set filters to shove stuff into different mailboxes
How to Use Eudora Mail Filters | eHow.com to keep the inbox size down.
Edit to add - I think the mention of maximum mailbox size '1 mb' is incorrect. A lot of single emails well exceed that! I suspect it is s typo for 'gb'.