Panorama postcards are a larger size, proportionally wider than average. Generally they are about 3.5 by 8.5 inches (roughly 9 by 21.5 cm), though of course they do vary quite considerably.
I've always considered them a problem for storage because I couldn't find an album with inserts that were the right shape. Actually, philatelic First Day Cover albums would probably do the job, but finally I found the sleeves I needed. so now I collect panoramas.
Panorama size cards lend themselves to, surprise surprise, panoramic landscapes! But as you can see from the examples below, they aren't restricted to that.
Some I've recently seen have a perforated side panel which could be detached to make the card a more standard size. It seems a pity to do that though. An example is the card on the right, centre. It is a picture of the Temple of Heaven in Beijing, and the side panel appears to be an explanation or description. It is all in Chinese so it's only a guess!