Some times using percentages for fluidity in layouts is tricky because you have to deal with containers and border-type things.

Look in to using viewport units and learn some thing about it on css-tricks and caniuse will show you how well it’s supported.

Essentially you can say:

<div style="height: 50vh;">...</div>

meaning a div element of 50vh height where 1vh is defined as the value of 1% of the viewport’s height.