Since you are scanning traditional black and white film and want to preserve the grain, you need to scan at the highest native resolution your scanner allows. The problem is going to be preserving the grain -- your scan (assuming it is sharp) will consist almost entirely of either black or white pixels. Shadows and lower midtones will probably survive downscaling fairly well, but upper mids and highlights won't -- those areas are going to consist of individual or small groups of black pixels with relatively small white spaces in between. A single pixel doesn't scale well, and a black-white-black-white alternation scaled to 50% is going to result in two grey pixels (with the discernible grain gone) or, if you boost the contrast to try to "recover" the grain, two black or two white pixels, depending on where the grey falls in the overall tonal scale.

You may find that creating a smooth greyscale image, scaling, then synthesizing the grain later is the only way you're going to be able to arrive at a reasonable file size. If you don't mind a larger file (and have a place to store large files) then you may be able to print the large files at a reasonable size.

Since you are printing a "true" balck and white picture (as opposed to a smooth greyscale), you can pretty much forget about the usual pixels per inch guidelines for printing. The normal rules of thumb assume that the printer needs a certain minimum quantity of dots per picture pixel to reproduce the tones accurately. You're not too tremendously worried about tonal range at every printed pixel -- each of them will be either black, white, or some transitional grey where the scan is getting the edge of a silver grain on the negative. One or two greys (or "light blacks" in printer ink-speak) ought to cover it.

That still assumes a relatively large print -- 8x10 or larger. If you go too small, you're probably going to have to sacrifice the real grain, create a greyscale image from the scan (requires some blurring), then use one of the grain-simulating halftone plugins in your favorite image editor to simulate the grain. (At small sizes, it's going to look like at least an extra stop of push if you want to see grain -- printer dithering can only get you so far.)

Added: Given the example you posted, there's something just plain annoyingly geometric about the grain effect from the methods that left a significant grain pattern (next neighbor and bicubic sharper; bilinear is somewhat less grainy, but it still looks too regular). Playing around a bit, I noticed that if you take one of the smoother methods (bicubic smoother in particular), then add about 20% random noise to an overlay layer set to 50% opacity, you can generate some organic-looking grain patterns in the overall image. It's worth a shot.