You can make a billboard-sized print from your camera. I will guess that you are either misunderstanding something about the printing process or Zenfolio is, because you can certainly print much larger than 24x16" with that camera.

You need to stop looking at file size (megabytes) and start start looking at image size (megapixels).  They are two different things.  File size has little to do with the maximum size print you can make.

1)  In ViewNX2, the "slider set to max" that you are referring to is for converting files to JPEG and only controls the amount of JPEG compression (file size), not the image size.

2)  When you crop in ViewNX2, at the bottom of the pulldown for crop sizes you should see two options that are the native image size for your camera.  One is for a landscape (horizontal) crop, and the other is for a portrait (vertical) crop.  If you select either of these options, ViewNX2 will do the crop and resize the image back to the original (uncropped) image size.

3)  You should check with Zenfolio (or Mpix) for what image sizes (or resolution in pixels per inch) they recommend for different size prints.  When I worked for ExposureManager (a Zenfolio competitor) we told photographers it wasn't necessary to use 300 pixels per inch beyond 11x14 prints, because of the increased viewing distance.  In fact, the same 2400x3000 pixel file you would use for an 8x10 (at 300 pixels per inch) could also be used for a 16x20 (at 150 pixels per inch).

Have you verified any of this by actually printing pictures, or are you going by what you are reading on the Internet, or what the software is telling you?

I print 24x36 all day long with 10mp cameras... stop listening to what the software is telling you.

I have found ViewNX to be rather unsatisfactory software, and much prefer Lightroom.

However, that aside, I would have expected your raw file to convert to a jpeg around half the raw size, not about a fifth of the raw size, so I wonder if there is a problem with how you are using ViewNX? 

A simple check might be to shoot simultaneously in raw with the large fine jpeg option on as well and then compare the file sizes.

My understanding is that on-line solutions for people / clients to order photographs from, such as Zenfolio and Smugmug, have a 'quality standard' where they limit the size of the photographs that can be ordered based on the pixel dimensions of the image. If you want to work around this you need to finde the required pixel dimensions for the largest size that you want to be able to be printed and then resize your image using appropriate software, for example OnOne Perfect resize ( used to be Genuine Fractals ). The OP does therefore need to listen to what the software is telling them for what they are doing and then find a workaround for it.

ViewNX & ViewNX2 are free and, in the case of ViewNX2, is updated for new Nikon cameras somewhat earlier than anything by Adobe et al. ViewNX2 also picks up any incamera image settings and interpret them as Nikon intends as opposed to Adobe's reverse engineered attempt. Trying to compare it to Lightroom is a little 'apples and oranges'. If I'm printing on-site at an event then I find ViewNX2 to be more than sufficient. For my general work my default software is CNX2, despite my having CS5. I've tried LR, LR2 & LR3 but have found those to be 'unsatisfactory' since I am expected to follow Adobe's way of doing things as opposed to deciding for myself.

JPEG will be smaller than raw, it depends on the compression value used as to how much smaller. This is independent of the software being used, just, as has been pointed out earlier, dependent on the value chosen.