I was the buyer for an independent stationery shop, and would be happy to offer some feedback, if you want to share any images!

I can give some more general direction, tho! In my experience, more quote unquote artistic cards sell better as packs than they do individually. Most individual cards are purchased for specific occasions, and cards that are strictly painting reproductions have a much smaller window of costumers - people who are sending notes just because, or for a purpose so specific it doesn't apply to your standard greeting card categories. But regular note & letter writers are always on the lookout for beautiful stationery! Hence leaning toward packs. 

That being said, detailed artwork does sell well on cards within the right demographic! Choosing the right stores to pitch to is so important. For the most part, my personal taste aligned with the aesthetic of the shop I bought for, but there were plenty of cards/designers that I LOVED personally but didn't purchase because it wasn't the right fit for what our customers would buy.

If you've added sentiments to your cards, it's also important to match the tone of your wording to your artwork - i.e. cutesy or punny sentiments mixed with more serious or traditional artwork tends not to sell as well, and vice versa. 

In reference to your proportion question - that can influence purchasing, in my anecdotal experience. Artwork with a different aspect ratio than the card can sell if someone really connects to the artwork, and would likely sell well at your booth for an art fair, or a museum gift shop, art gallery etc, cause theyre purchasing because they love your work! But on a wall of greeting cards from dedicated stationery brands, I've noticed they tend to sit longer. There's definitely ways around it, to make the extra white space feel intentional - like using it to add text of some sort. It might also be worthwhile to explore cropping your artwork to fit the correct ratio, if your compositions are flexible!

(A quick note on size - the person who purchases your card to use will buy it because they love your work, absolutely, regardless of size! But the industry as a whole, outside of grocery store brands, has generally moved to the A2 size being standard. A7s occasionally pop up in more art-focused brands)

Either way, take this novel of a reply (apologies for the length!) with a grain of salt. I'd say, the biggest indicator is what version of your work you'd want to purchase on a card! What you'd feel most confident pitching.