Welcome to the Cake Decorators Q&A

How do I adjust the quantities of Mrs Jones vanilla sponge t make a 7″ cake

Hello laine

Here http://www.cakeflix.com/blog/how-to-work-out-what-size-cake-tin-to-use is where to look. You can use either the ready reckoner or the link which will take you to the cakeOmetre. Either method is easy to use.
Hope this helps.