My cot mattress is 130 x 69 x 12 cm. Is it safe to purchase cot sheets and mattress protectors that are bigger (132 x 77 x 19 cm) or is this a safety hazard?

Red Nose recommends that baby’s head & face remains uncovered when sleeping.
https://rednose.org.au/article/sleep-baby-with-head-and-face-uncovered
It is important that any bedding is not loose.
Sheets should be able to be tucked in so they so not become loose.
Mattress protectors should be a tight fit & not padded.
..... always make sure that the waterproof mattress protector is strong and a tight fit.
Never put soft bedding under the bottom sheet, such as a sheepskin, as this makes the sleeping surface too soft.
https://rednose.org.au/article/what-is-a-safe-mattress