Question about image scale on 9 Slice and resolution (Beginner)

Hello,
I am an Artist and very new to Unity, and mainly use it to check my export Images to my Dev now.We like to have a 9 Sliced Dialog Background with round corners. I started layouting and exported my graphics, now the scale is kind of weird, and i basically just need to know how I can avoid this, and on which resolution I have to work. Apparently the resolution is different, but my Dev just keeps on saying that we are not working on a particular resolution so I dont find my factor x to create my graphics in the correct and directly usable way.
If I change the scale to a smaller size than 1 like 0,3 it looks more likely how i wanted to have it, but my dev does not want to change the scale, and thats okay for me, just need to figur out how to create it right.
If someone finds the time to explain?
Thanks

Nobody?

I try a second attempt.
My Interface, which contains 9 sliced Elements looks different depending of the resolution (see mock in attached file). How can we fix this in a professional way?