Are there limits to the size, resolution, etc. for renderings done with irender nxt for SketchUp? For a competition, I have a specific request for a 24"x16" image at 300dpi.
No, there are no size limits.
To render to that size, set the render size to 7200 x 4800 pixels and print the result from a program that will let you set the resolution to 300 dpi, like PhotoShop.
Now...it will take quite a while to render to an image that size.
7,200 x 4,800 is 34 million total pixels. This is quite possible, but it will use more tha 2GB RAM - so you should have a 64-bit processor and at least 4GB ram. It will take a while. For instance, if it took 10 minutes to process a 1,000 x 1,000 - 1 million pixel image, it would take 34 times as long - 340 minutes which should be doable overnight. So you should do you renders at 720 x 480 until you like the result, ant then go for the large rendering.