Can I ask why you want to change the menu dimensions? That is, is there something wrong with the way it looks now? You first wanted the height to be 108px, but now you want it to be 98px.
The width is going to depend on the class that you assign to the menu wrapper. Go to Appearances > Montezuma Options > CSS Settings and read about the CSS grid system. It is that 12 column grid system that Montezuma is based on, and it allows your site to be responsive. Pay close attention to how to how the column classes have to add up to 12 for each row DIV.
Then you're going to go to Appearances > Montezuma Options > Sub Templates > header.php. This is the virtual PHP template that determines what the header looks like.
Right now, your heading has a row with two DIVs in it, one for logo-area and the other for menu1-wrapper. The logo-area DIV has a class of col5 and the menu1-wrapper DIV has a class of col7, which is good because 5 + 7 = 12. If you want to make the area for the menu wider, then you just adjust the class assigned to the DIVs. If you are using a 960px grid (which is the default) then each column is about 80px wide. col7, therefore, is 560px wide (7 x 80px = 560px).
You say you want to make your menu 723px wide. That would correspond to about 9 columns (723px / 80px = 9). So you should try changing the class for menu1-wrapper from col7 to col9. To keep the number of columns at 12, though, you then need to adjust the logo-area class from col5 to col3, which means your logo area will be smaller.
If you want, you can just put the menu and the logo on separate rows, then you can make them as wide as you want. But study header.php and see how it formats the look of your header.
If you have other questions about the Montezuma theme, you can post them on their support forum.