Hello,
Use the below custom code to change the width/height/font-size of the q-ty on the single product page
.et_product-block .cart .quantity, .et_product-block .quantity-wrapper {
height: 50px;
}
.et_product-block .quantity-wrapper input.input-text {
width:50px;
font-size: 1.5rem;
}
Regards