The G4 is manageable in 1 hand. My hands are not enormous but after a week, it felt just like my Nexus 4.
I would not use it one handed while texting walking down the street. Two hands is much safer.
Give the S6 a try as I have heard nothing but good things. The battery is not good at all but you can purchase an inexpensive charging brick, like I had with my Nexus 4. It's annoying yes but manageable.
As per the G4's battery life: I have bought 2 spare OEM batteries and a charging cradle from supercosmos on eBay. Its overkill but i will not have to worry about battery life for however long i keep this phone. If you live in the USA, take advantage of LG's free battery promotions as well.
Battery life is amazing compared to my N4. The screen does use most of the battery so if you're comfortable with around 33%-50% brightness you will be fine.
Good luck, both phones are great.
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