Hello,
Add the below code to page custom CSS in the Elementor settings while editing your page (if you want to apply it for this page only) or to Theme Options > Custom CSS if you want to change that for all the counters on your site
@media only screen and (max-width: 768px){
.products-list .product-sale-counter.et-timer {
font-size: 10px;
}
}
Regards