Can I say: Her eyes got bigger and the voice got shakier.
is shakier a word ?
Yes.
https://www.google.com/search?source=hp&ei=cm8VXZaNHtiltQaboLi4Bw&q=shakier&oq=shakier&gs_l=psy-ab.3..0i70i249j0i10j0j0i10l3j0l2j0i10l2.1867.5763..6016...0.0..0.130.905.0j8......0....1..gws-wiz.....0..0i131.3i2nNZ6aluI