I think you will find the answer is not too much. 6MP is going to limit you on final output. Epson needs 360 dpi for ideal output and Canon 300 dpi. You can try any of the specialized software tools and uprez your shot, like:
Photozoom Pro
Genuine Fractals, now part of On-One photo suite
LR's uprez
CC 2014 uprez that preserves details.
If you don't use LR for the print (which does a nice job of uprezing to 360 dpi in the print module), then any of the the tools I listed will work better than allowing the print driver to make the uprez. If you are printing to Epson, and send a 300 dpi image, the driver will still uprez the image to 360 and the driver is using nearest neighbor, possibly the worst algorithm there is. Another option is Qimage (PC only) as again, you have an very good tool that can provide the correct uprez when making the final print.
A lot also depends on the camera and lens that were used and how sharp the image is. Is it a macro or a image with a lot of small details. I believe you will find a macro shot will give you a larger overall uprez opportunity.
The largest I have been able to get a 6MP output from a Nikon D2x 6MP camera is about a 20 x 30 and that was pushing the file quite a bit. The D2x had a strange layout in pixels so you were penalized from the start.
I also feel that the printed results will improve with the best possible 6MP output and raw conversion, not working with an in camera jpg.
Paul