Hey, the live portfolio (preview) has about the same dimensions as a gig image, however the actual preview which pops up in the lightbox, shows much more.
Take this gig, for instance: https://www.fiverr.com/dynamic_website/design-wix-business-and-ecommerce-website?context_referrer=search_gigs_with_recommendations_row_3&source=top-bar&ref_ctx_id=fd30f5d44c39d6bda28dc55756b92da1&pckg_id=1&pos=2&context_type=auto&funnel=fd30f5d44c39d6bda28dc55756b92da1&seller_online=true&fiverr_choice=true&imp_id=22c04681-bc50-4e80-82d5-0e55b4cc3b2c
If you look at their live portfolio, and flick through, you'll see a few things.
1) The preview at the bottom is squashed
2) The actual preview in the lightbox shows a long portrait shot of a website.
A the moment, there's no way to see the lightbox preview, which means it's a little bit of guesswork / experience with getting it right!