上传和查看包含文本的图像的最佳方式

Question

I'm creating an app which published E-Paper. The problem is that my E-Paper pages are in JPEG format of high resolution (say 2200*4200, 6 MB). Now, I need to upload it to server in enough size that :

1. I don't get 413 error code while uploading.
2. When user see the image on their mobile, the text are enough visible.

How can I crop the image perfectly enough for a clear view. Maybe in resolution 1280*720 will be enough?