1. I'm not aware of a Super Mario Galaxy 2 HD texture project. We don't have one hosted here on in the HD Textures forum anyway.
2. Whatever you're comfortable with.
3. There are some good tutorials on N64Redux. Google for 'generic' (HD) texturing and you should find more tutorials.
4. That depends on the textures. For the best compatibility between all back-ends and for the DDS format, you'll want to scale your textures in a multiple of 4. So if the original texture is 256x256, then you can create your better texture in 512x512 for example. You can however use 'weird' sizes with PNGs such as 1012x841, but be warned that it will cause some slowdown and incompatibility.
Last but not least, don't go way too overboard. You don't need 4k textures for every little thing.
2. Whatever you're comfortable with.
3. There are some good tutorials on N64Redux. Google for 'generic' (HD) texturing and you should find more tutorials.
4. That depends on the textures. For the best compatibility between all back-ends and for the DDS format, you'll want to scale your textures in a multiple of 4. So if the original texture is 256x256, then you can create your better texture in 512x512 for example. You can however use 'weird' sizes with PNGs such as 1012x841, but be warned that it will cause some slowdown and incompatibility.
Last but not least, don't go way too overboard. You don't need 4k textures for every little thing.