Create and Export 2048 Textures in Zbrush
In order for this to work you must have your UV's laid out on your complete high res model. Once complete, keep model on the highest resolution to create the textures. If you follow these instructions you will have textures that have a 2048 dimension.
Step 1.
Go to drop down texture, and adjust the Width and Heighth to 2048, (be sure the color is set to white) and click new. As shown above.
At this time you should have a white texture that has the dimensions of 2048. To make sure hover over the texture, this will give you the dimensions in the bottom left corner. As shown above.
Step 2.
Go to your Tool bar and under Texture click on Txr>Col. As shown above.
The white texture should be added to the model. At this point you are free to paint but before you do so be sure that you turn off Zadd and or Zsub.
Step 3.
Once you are satisfied with your texture, SAVE and then proceed.
Go to the Texture icon box above the Material icon, and you should have the current 2048 white texture. As shown in the image above.
Click on that and your model should go all white. Don't worry you did not lose your texture. As shown above
Step 4.
Go to your Tool bar, under Texture click on Col>Txr. Your painted texture should be applied to the model and you should see the texture laid out like your UV's.
Step 5.
At this point you can export your Texture without needing to make any changes to it in another program. If you don't save it as a TIF it will save as a PSD. In that case, you will have to re save it in a paint program for it to be recognized in Most 3D programs.
I hope this helped you out with Creating 2048 and Exporting 2048 textures in Zbrush 3.1