@Kaimi ,
hi, recently I bought a device specifically for use with Pleco (and Chinese learning in general).
It's Xiaomi Mi Max 2 phablet with 6.44" display (my first "totally Chinese" device

). Its size is enough for comfortable use for the purpose, it's lightweight, has very good performance and costs just about 200$-300$ (depending on the seller and support package).
For eyes protection you can use Pleco's night mode or Xiaomi's Read mode, wich reduces blue-light emission and is very comfortable (making a screen looking like a real paper). Though display itself obviously is far from high-end, but is not bad. And the device has outstanding autonomy - several days of usual tablet-mode usage with more than 20 hours of active display. It has quick charge function (full charge = 2 hours, 60% = 1 hour).
Today Xiaomi announced Redmi 5 Plus with 5.9" display and with the same SoC - it will be even cheaper and should be enough for learning purposes, though I don't know all details.
Just a tip

I think phablets are best devices for learning purpose - they're big enough but still compact and lightweight. And you still can use them for making a call!!

))