According to the codex, you should be able to split the XML file into pieces and import them separately. Here is how I would do it.
The WordPress XML export file is in a format they call WordPress eXtended RSS (WXR), and looks like this (some lines left out):
<?xml version="1.0" encoding="UTF-8" ?>
<!-- several tags about your blog, including title, link, description, etc. -->
<wp:author><!-- a list of your post authors here --></wp:author>
<wp:category><!-- a list of your categories --></wp:category>
<wp:tag><!-- first tag info --></wp:tag>
<wp:tag><!-- second tag info, etc. --></wp:tag>
<!-- first blog post here -->
<!-- second blog post here, etc. -->
You'll need to split this into at least 6 different files to stay under your 8MB limit. Copy the file until you have 6 or 7 copies of the file. Then in each copy, remove some of the
<item> elements (posts) until you are under 8MB. So each file will have identical information in the first part of the file, and will only differ in the
<item> section. Make sure you don't include the same post in more than one file, or it will get imported twice. Then import each file, one at a time.