Or explained a bit differently,
that sRGB you see is what _you want it to be_ when you're done editing the RAW file and make it into a real photo with pixels. If you want it to be something else, pick something else :).
As stated above, raw files don't have a color-space yet, it is 'undefined'. The job of a raw-converter is to take that data and map it to real color, and while doing so choose in what profile you want that color to be.
Affinity just defaults to sRGB but you can pick Adobe RGB or ProPhoto or whatever you want.
About the size: As you may know or not know, an image sensor (in your Nikon at least) is working in _groups_ of pixels. The Bayer filter.
Now, at the sides of your image, you have pixels that are only half a group, or less. So what is a raw-converter to do with those edge-cases where it does't have a full group of pixels available to make up color? Most camera's and raw-converters crop those edge cases, but Affinity (and dcraw and some others) choose to 'make something up' at the edges to not throw away the few pixels at the side.
As example, my Sony DSLR gives files of 6000x4000 with in camera jpg, and also when using raw-converters like DxO and CameraRaw. But the resolution is actually 6008x4024 or something like that. So most converters crop 24 pixels at the sides and 8 at the top + bottom to get 6000x4000. Others don't. Perfectly normal.