You may or may not have seen my HQ4X basic texture pack. I have now made the process that I used into a convenient tool. If you insert a standard 16x16 texture pack, the tool will return a 64x64 texture pack.
Good luck!

Instructions (available in readme file):
0. Unpack zip
1. Put .xnb files in folder "Input"
2. Run XNBExport
3. You now have .png images in folder "Output"
4. Run PokeHQ4X
5. You now have large .png image in folder "Scaled"
6. The exceptions file is automatically generated
Notes:
- Some artifacts will occur in the textures.
- The overworld sprites (of the pokemon) have to be removed from the exceptions file.
- Keep in mind that if you have a lot of larger textures, the game might run slower.
- If you want to enlarge an existing texture pack, put it in the folder "Output" and do steps 4 and 5.
Credits:
PokeHQ4X (by Antony Löbker)
XNB export (by Nilllzz)
hqxSharp (by Tamme Schichler, Maxim Stepin and Cameron Zemek)
Good luck!

Instructions (available in readme file):
0. Unpack zip
1. Put .xnb files in folder "Input"
2. Run XNBExport
3. You now have .png images in folder "Output"
4. Run PokeHQ4X
5. You now have large .png image in folder "Scaled"
6. The exceptions file is automatically generated
Notes:
- Some artifacts will occur in the textures.
- The overworld sprites (of the pokemon) have to be removed from the exceptions file.
- Keep in mind that if you have a lot of larger textures, the game might run slower.
- If you want to enlarge an existing texture pack, put it in the folder "Output" and do steps 4 and 5.
Credits:
PokeHQ4X (by Antony Löbker)
XNB export (by Nilllzz)
hqxSharp (by Tamme Schichler, Maxim Stepin and Cameron Zemek)