Canva for Cover Page images

Hi There, i would love any design tips around creating cover pages for Microlearnings in Canva. 

What kind of document type do you use? 
A4 size, landscape, Instagram post size?

Any tips would be great as ive seen some great content being made and i was playing around with one image on a blank landscape page saved as png file but wasnt sure where to position the image on the page because of where the heading shows etc and then i found it looked good in desktop view but not mobile...

Hope this makes sense

Thanks Kylie