As a manga reader, Kobo Forma or Aura ONE are about as small as I would recommend for manga. In addition to physical size you want good contrast and relatively high dot pitch. Here's my comparison of my Kindle DX and my Forma for reference:
https://www.mobileread.com/forums/sh...4&postcount=32
I'm not a fan of the Android readers. They run very obsolete verisons of the OS with all of the problems this entails. If you're thinking about a larger device then get an iPad.