Shopify's video size limit is 1 GB per file, 10 minutes long and up to 4K resolution, in MP4 or MOV.
Shopify-hosted video accepts files up to 1 GB, up to 10 minutes and up to 4K resolution, in MP4 or MOV; the 20 MB cap applies to images.
A Reel exported at 1080x1920, 30 frames per second, 20 to 30 seconds is usually well under 50 MB, so the limit is rarely the problem.
Bigger files are not better on a product page. Shopify transcodes every upload, and a smaller source processes faster and looks the same.
Video apps set their own upload caps, for example SwipeReel allows 100 MB per upload on the free plan and 500 MB on Pro, which is enough for any Reel.
Shopify-hosted video is limited to 1 GB per file, 10 minutes in length and up to 4K resolution, in MP4 or MOV format.
The full guide has the formats, the deadlines and the worked examples.