Maximum filesize or maximum testcases for an XML import?

I'm trying to import a rather largish XML file (13mb) but keep getting an error when I try:

(bah, how to I upload images to the forum?) ... here's what is says in a popupish window:

"An error occurred during the last operation or your installation is currently in maintenance mode.  Please try again or refresh the current page."
(With an "OK" button.)

If I bring the size of the file down (and hence, the number of testcases) I can do a successful import.

Is there a size or testcase limit per import?

Thanks.