That is what it is and you have reached its max. To get more you have to go to programming and vba. See the Diane Chapman's articles listed in the additional materials section of Making Forms with Microsoft Word. (oops, just checked and those articles are gone)

Here are solutions I found doing a Google search for "dropdown field limit word"

http://www.vbaexpress.com/kb/getarticle.php?kb_id=968
http://www.wordbanter.com/showthread.php?t=100803
http://vbaexpress.com/kb/getarticle.php?kb_id=5

Jay Freedman, Greg Maxey, and fumei are all proven programmers.