Hello,
You use the classes to apply changes. If another element has the same classes then changes will be applied for that element too. You can add the custom to reduce the size of the other prices
.product-information-inner .rightpress_product_price_live_update .woocommerce-Price-amount, .product-information-inner del .woocommerce-Price-amount {
font-size: 20px;
}
Regards