Can't make it resizable for all devices

Hi, guys.

I’m developing a math game in which I have a calculation area and a few operator buttons beneath it, and I’m having a real hard time styling them.

You can see hoe it looks like right now below. At first I couldn’t find Ionic components that would fit me perfectly, so I thought it would be easier to get any component and personalize it by editing most of the CSS. But I’m having second thoughts about it, specially because I can’t make it resizable (including the font size). I mean, I believe I would eventually make it work on the serve preview, but I wouldn’t know how to make it fit all devices.

How would you guys make it work? I thought about the Grid component, based on flexbox. Is that the way to go?