Vector images are stored objects (lines, squares, circles, etc) so when resized the program can easily figure out what it should look like. Raster images are just a matrix of pixels so when you resize, the only option is to start duplicating pictures. It won’t know how to make a circle still look like a circle, for example, because it doesn’t know that there’s even a circle so edges tend to look jagged and things just off in general. Vector images can have raster images embedded within them or objects that, though look fine individually may not look great resized. A stick figure of a human, for example, will look off if it becomes narrow relative to height. A tall image may not look right if resized to be a wide one. A small modification like making it 10% wider should be relatively well tolerated since it avoids the approximation effects of a raster image.
tl;dr Find the dimensions you need then ask designer to create it to those exact dimensions
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