Scaling graphic on custom box dimesnions

So recently i started working on dimensions. I wanted to make a custom size box and add my design on it. I grabed a cube box, resize to requred proportions , but when i want to put a design on it, it scales acording to the scaling i made from the cube box. I want my  design grpahic to go from one side to another, without scaling issues. I notice this problem ocurs only if i mess around with box proportions. Is there a way to ignore the custom proportions while aplying a design?

Sincerely,

Marius