Very confused on styling a form

i got a form and i put a border around (to beautify it) im trying to modify the dimensions of it. im trying to make the width and the height bigger, while keeping it responsive, without heigh/width mods its nicely responsive, when i start adding h/w to the formblock - contact form- and text areas it changes it to non responsive. what is a good solution and best practices for this kind of dilemma?

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