The Threads API documents images from 320 to 1440 px wide, JPEG or PNG, up to 8 MB, converted to sRGB. That extra width over Instagram's 1080 px is visible on large phones, so the default preset here is 1440×1920. If you cross-post to Instagram, 1080×1350 keeps both apps happy.
Turn on Upload highest quality in Threads under Settings › Account › Media quality, otherwise the app downsizes before upload on mobile data.
Try it on your own photoDrop a photo at the top of the page. Ten seconds later it's sized, sharpened and ready to post.
Compress a photo