Lightroom crashes when attempting to change output pixel dimensions

Whenever I attempt to modify the export dimensions for a jpeg lightroom freezes/crashes (permanent spinning pinwheel and force quit reveals that lightroom is "not responding"). this happens 100% of the time. below are the steps that reproduce this problem:

1. select "export" from the "file" menu

2. select "For Email (Hard Drive)" from the lightroom presets 

3. try to enter a different number for the width and you get the spinning pinwheel

i have attached a screenshot showing the window just before i hit the "return" key.

this happens both from my ibook pro and my desktop mac. both running current mac os and current lightroom.

any thoughts? please help... thanks!