Planning to design a banner, let's say, 800x150 px.

It will expand upon mouse interaction and the size will expand to 800x500 px

To do so, should I start with a project having the size of 800x500 ?

If I do so, the banner will be having that much space (800x500 px) when I place it on the website (presumably). However, what I want is, having a space of 800x150 px and when it expands, it will expand over content. So, at the very beginning, when there is no interaction with banner, it should occupy 800x150 much px area.

How can I do that, is there a technique for that purpose ?

Thank you.