Bad news for brands looking to capitalize on Instagram's new variable size carousels: if the images in the carousel are full-screen size, then they don't qualify for boosting.

Last month, Instagram released a new feature that allows users to upload carousel collections of mixed-sized images, giving them yet more ways to compose carousel posts in-stream.
Which was a greatly needed update but now people are discovering that if an aspect ratio image of full-screen size is included within the new format, the post can't be boosted.
if you go to boost a carousel with a full screen image, you get an error saying that this can't be done.
This could ruin your revised plans for a carousel.

That does appear to be the only limitation on the new carousel format at this point, and it is largely avoidable, should you really need to promote your post. However, it might serve as a limitation on your usage of carousels for promotions and the overall value of this update.

We have reached out to Instagram for more details on this error message and if the restriction applies to all carousels with full size images.