Is there a way to make
No you cannot set the size of the font in a percentage relative to the size of the page.
em is based on the size relative to how the font would normally render in 16 point.
If you'd like your page to scale up and down and keep some sort of scale to itself, the boxes and the font, then setting it out in
em would be fine.
That method will allow for the scaling of fonts and boxes of the page to grow at a relative size to one another and hopefully not have things falling out of bounds and borders.