Image sizes in Storyline 360

I am a total novice, trying to develop my first Storyline 360 course.  I have done a little bit of work in Rise.  I am not very technically minded, and just entering eLearning design for the first time.  I have asked our designers to design some images for me to use and they need to know the size specifications, both ratio and image size - MB.  I know for Rise it is max 5MB for images.  Is it the same for Storyline?  What happens if the image is much bigger or smaller?  What is the best practice/your advice for what size image I should use?  Also what is the best practice re. ratios/image sizes?