The sites' max pixel size seems to be around 1200
Google Photos stores at around 1100 so if you have that you can sync to your mobile and it updates as soon as your phone hits wfi. Then you can just copy and paste into a program so you can name them and so on.
Irfanview is a free package that allows you to resize any image plus crop, zoom etc.
Its for Microsooft machines only though, maybe for Mac but definitely not for Linux