You can hide and show interface elements when selecting various reservation options.
You can do this by using a reaction of type Add class.
You will also need a little experience with HTML.
Let's do it with the resource "Snowboard rental".
In the resource configuration, click Add reaction.
Enter a name, e.g. "Show size".
Select type Add Class.
Make up a name for the CSS class. e.g. show-size.
Then add a condition.
Select the condition that one share of the "Snowboard with boots" price option is selected and save it.
Now, when choosing this price option, the CSS class show-size will be added to the reservation form on the widget.
Now you need to write a simple CSS in order to hide the field from the form.
Go to the widget and pick some available time.
A reservation form will be shown with a field we need.
Open the page markup and find the id of this field.
Right-click on the field and select View the code.
In our case, the field id is field_49225.
After that, open the widget settings.
Go to the Design panel.
Set the Custom CSS style checkbox.
And add the following css:
#field_49225 { display: none; }
.show-size #field_49225 { display: block; }
Check the result on the widget: