We don’t worry too much about size on Facebook for organic posts. The user view varies so widely by device (or even same device type!). 

If it bothers you, instead of posting to both channels at once, use Sprout to schedule to Instagram first, and then, using the “Schedule and Duplicate” option, crop your photos to square for Facebook. 

I don’t feel sizing matters to much on Facebook, but I normally use 1x1, 4x5, or 4x3 on Facebook and I haven’t had to much issues with those measurements. 

Agree with the above that the sizing not mattering on Facebook. I do use the ‘Schedule and Duplicate’ options but instead of cropping. We have created templates on our Canva for Instagram 4x5 and the Facebook Square in order just to drag and drop images to better fit both platforms when necessary. 

We’ve been doing carousels 1:1 for FB, but it’s because we also share that same size on YouTube so it works with our workflow. Given how much it can vary by device, I’d go with whatever is best for your workflow unless you’re getting complaints or seeing a decrease in engagement. In that case, you could experiment with 4:5 vs 1:1 to see what gets more attention.