Size guide block: create a page in Online Store → Pages with your size chart, then choose it in the block. It opens in a side panel.
Specifications block: enter one row per line as Label: Value. Connect the field to a product metafield to show different values per product.
Product details section: image rows with a label, heading, text and measurements separated by |, for example Length: 32 cm | Weight: 120 g. Connect the fields to metafields for per-product details.