Well, there are a few limits I can see. First, Wikka Wiki has a 2MB limit, but it also looks like the underlying hosters has a 2MB limit as well. You have to change the "php.ini":http://www.bluehostforum.com/showthread.php?t=12687 files in the wiki /uploads directory to break this.
"Bluehost":http://helpdesk.bluehost.com/kb/index.php?id=124&mod_id=2&x= explains how to do this. Note that this thing is actually not quite right, you have to put the php.ini in the root of your /public_html on bluehost.com. It doesn't seem to work to just have it in the directory where it was executing. I also set max_postsize = 110M as well.
# Locate the php.ini file in the folder your upload script resides in. If none exists, create a new text file called php.ini
# In the php.ini file, locate the line called 'upload_max_filesize = 2M;'
# Modify that entry to read 'upload_max_filesize = 10M;', or whatever your script requires. (M = megabytes)
Put another way, stick the following into php.ini in the script files. On Wikka wiki, thats the /actions folder:
upload_max_filesize = 10M;