iphoto thumbnail rows? How do i change them?

I managed somehow to reduce my normal row of five images going horizontally across the screen to just three however i cant find a way to get the normal five back again. I have been into 'view' then 'thumbnails' but all the options to adjust the columns are greyed out. Any ideas please?

iPhoto thumbs



Move the slider on the lower right hand corner of the iPhoto window.

Hope this helps!