How to stretch my Page Template`

Hi

Im using one iframe from external site, see below:

https://i.gyazo.com/0e7d3b18a479d97acc148d890cb59682.png

This iframe looks lige bigger than my template, I using showcase template, I would like to stretch this template in order to fits my iframe

https://i.gyazo.com/bc54a94a502906283d830cc8801ee569.png

anyone can help me? thanks