The cover should be colourful yet not childish and attract people that like odd adventures like Alice in Wonderland, The Little Prince, the Wizard of OZ etc.
The book will be A5, the spine width is not yet 100% final as the number of pages and paper thickness is not fully final yet but this will come very close:
File size needed:
33,6 cm + 1,6 cm for the spine x 24,5 cm
True size of the book A5:
30,6 cm + 1,6 cm x 21,5 cm
Some addition info:
The train they are travelling in is in ICE fast speed train.
The train falls straight through the earth and lands on the other end of the globe in the pacific ocean, where there is an imaginary island called Sonowhere. If there are any further question just let me know!
The images you find attached are illustrations from the book.