LaserSoft Imaging

I have just updated to Silverfast 8 and am undertaking a project involving scanning slides. In preparing for a scan, I have selected a Slide 35mm frame via the menu at the top of the screen, however, prior to scanning, the dialogue box (on LHS) resets the fram size to 32.54mm x 22.25mm. Why does this happen and how can I set the size to 36mm x 24mm, as I will resize and crop where required at a later time in Photoshop.
Any assistance would be appreciated.

Dear Graham,

you can open the expert-dialog with the gear wheel and type in the value you need.

Best regards
Artur Möller