With the wiki markup table you are limited to the normal formatting.
For more complex formatting you can enable the table related html tags (e.g., table, td, tr, th) in the HTML filter. This can be found in the administration console under Spaces > Settings > Filters and Macros.
If the HTML filter is off, all html content is passed through, however this can be dangerous. Edit the settings for the HTML filter and add the table related tages to the allowed tags.
Any set width, colspan, etc. will be rendered as is by the browser.
Watch out for this known issue when editing the content or you may lose your submitted html formatting: http://www.jivesoftware.com/jive/thread.jspa?messageID=119150𝅮