We use the theme for a bike rental in Spain.
The thing is that we have a several bike sizes, like S, M, L, XL.
Is there any way to add an option to select the size of the bike with an dropdown menu or value field where they can fill-in their body length.
There is no such possibility in the theme. You will have to add each size as a separate item and specify its quantity in stock (the “Stock management” option must be enabled).