Thanks for the replies and advice everyone, decided to use the 3cm side. Wonder if the 2cm difference between the recommended 2in(=5cm) will make a huge difference in the shape of the impressions?
And I certainly hope I do not freak out!
Don't want to risk sending the set back for multiple refits.
Every person is different. Some have had excellent results with just a closed mouth impression. Some (like me) need a really wide open impression. You won't really know until you try it for yourself.