How to adjust picture, mobile?

So I want to adjust images so they are just as big as the screen of my phone

Screenshot does not help because then you see the keys and who cut it off, it is then again too small

Do you know how this works?

Have a Huawei P 20 lite

Maybe Aviary

I can't find it in the Play Store

If you want to have this picture as background picture you can search in Goolge for different sizes of this picture

Look, maybe: InstaSize

You can cut the appropriate size with an image editing program, so adjust the size of the image beforehand, crop, save and then transfer to your smartphone.