Optimal Story size for Mobile (portrait only)

Looking for advice from folks familiar with mobile development in selecting the best story size setting for a mobile, HTML5 only, portrait only, authoring environment. I am working with a team developing a mobile app (phone only).  As we are testing Storyline interactions we find that a range of story sizes seem to work fine on mobile phones.  Since we will be developing dozens of custom interactions, want to select the best size from the start. Is it best to go with the maximum size 1080X1920, something in the middle, 750X1334, or the smallest size that works?  Will going larger cause longer load times? Does the larger size offer any advantages? What other considerations are there? Thanks for any advice on this!

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