By default, ArtStation will resize images to fit within the user's viewport. This is overwhelmingly how users want to view your artwork.
There are some cases when you want to show your work larger. E.g. when you have a very tall image and you don't mind making your users scroll to view the artwork.
When uploading artwork, you can specify how you want the image to be displayed. By default, it will fit in viewport (this is the recommended setting), but you can select "Maximize width" to make it show at full width.