This depends on the device (i.e. on the software). Some display fonts at the same physical size on 5" and 6", so they show less words per line on the 5" screen. Others display fonts at the same number of pixels on 5" and 6", so they show the same number of words per line but the font is physically smaller on the 5" screen.
In the end, you need to find a comfortable font size and it would not be surprising for this to end up with fewer words per line (and per page) on the 5" screen. Note that you can usually rotate to landscape. I never do this on a 6" screen, but I sometimes do on a 5" screen (I use a relatively large font).
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