You can use these CSS code to change Product Title text size on Mobile View Only.
#1. Product Title in Product Detail Page
@media screen and (max-width:767px) { h1.ProductItem-details-title { font-size: 18px !important; }}
#2. Product Title in Shop/Category Page
@media screen and (max-width:767px) { .products.collection-content-wrapper .grid-main-meta .grid-title { font-size: 10px !important; }}
#3. Product Title in Cart Page
@media screen and (max-width:767px) { a.cart-row-title { font-size: 10px !important; } }
#4. Product Title in Product Block
@media screen and (max-width:767px) { .product-block .productDetails .product-title { font-size: 10px !important; } }
#5. Product Title in Summary Block
@media screen and (max-width:767px) { div.summary-item:has(.summary-price) a.summary-title-link { font-size: 30px !important; }}
#6. Product Title in Quick View Lightbox
@media screen and (max-width:767px) { .sqs-product-quick-view-lightbox .ProductItem-details-title { font-size: 10px !important; } }