Do you mean the size of the post on your actual page when published? Because the size of the post box is in 'Writing' not reading and refers to the size of the box you type your post into.

I think you most likely have excerpts turns on which is on by default in the newer versions of Atauhalpa.. no idea why mind.

Go to 'Atauhalpa Theme Options'
Find 'Configure Excerpts'
Set 'Posts or excerpts on HOME page?' to 'Full Posts'

Like..



Hope that helps.