Pixel art generally responds extremely bad to any scaling. Just a small size adjustment will make pixel art appear either blurry or distorted, depending on what scaling algorithm your engine uses. For that reason it is important to make sure that the graphics are designed in exactly the resolution in which they appear in the finished game.
Here is a pixel-art sprite from one of my projects, interpolated with linear interpolation and nearest neighbor interpolation. As you can see only the original size looks well:
When stepless scalability of graphics is important for your game concept, you should consider to use vector-based graphics instead of pixel-art. Just keep in mind that the hardware requirements can become a lot higher when the vector graphics are not specifically designed to be fast to render. When your artist comes from a general visual design background with not much game development experience they might not know what they need to do and avoid to ensure this (it also depends on the platform and technology you use. Different vector graphic rasterizers have different strengths and weaknesses).
Another option when you want scaling is to go for a retro-look, design very low resolution sprites and upscale them with nearest neighbor. Nearest neighbor preserves the "blockyness" which is usually undesirable, but preserves the aesthetics of 90s style pixel art on high-DPI screens. This usually only works when you upscale by a large amount. When you downscale, most other algorithms are preferable to nearest neighbor, even on pixel art. Here is a very small sprite from the first Final Fantasy game upscaled a lot with nearest neighbor interpolation:
As you can see the retro aesthetics are preserved, and the image quality doesn't suffer even with a very uneven zoom factor. But this only works because the original sprite is so tiny and the zoom level is so high. Also, this look is not appropriate for every game.