Well, this depends on the quality of your camera. 6mpx is low on current standards, but it could work.
Look at this graph. This graph represents the relation between the viewing distance and the ppi.
I'm asumming that the maximum size of the print is somehow simmilar to the viewing distance.
[![][1]][1]
So you can export at 35 ppi and you still will do fine. This is a pixel of less than a milimeter. If you do not want to push it that much a 50-75ppi will be ok.
But remember that the texts are always better if you work them in vectors. [1]: https://otake.com.mx/Foros/Grafica1Eng.png
