"will my image be restricted to a smaller size or will I still have my full range of size like I do with P4 rendering?" There's no doubt that Firefly demands a bit more from your system than the P4 renderer. P4 may also be faster, but that's primarily because it's "doing less". I rarely need to render something beyond 2400x1800, but I have successfully completed a couple renders in Firefly that were over 3000x3000 each. It really depends on your system and how much geometry is in the scene you are trying to render. If you find you're having trouble completing a very big render, try to lower the bucket size, which will require less memory to be used, but will also take longer to complete.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.