Regarding sizes - Newborns in Need has a good chart to show approximate preemie measurements at various weeks of gestation. This can help give you a sense of how tiny some of these little ones are, and help you judge sizes for gowns and wraps.
And if you'd like to use a pattern, some of the best I've found are on the Bridging People and Places site. Here are some I finished this weekend:
These gowns are all less than nine inches long, made from the small-size infant pattern on the link above. I used the option that allows for a little gathering at the neck. It's important to leave the back of the gown completely open - just add some ribbons so it can be tied closed. These fragile little babies can only tolerate the most gentle of handing, and an open-backed gown allows them to be carefully dressed without damage.
Several people asked about sizes for bereavement blankets. They can be as small as 9 or 10 inches square, or up to about 24 inches square. Here's a quilt I just finished that I'll take to the hospital with the rest of these things tomorrow. If you'd like to make a blanket but aren't sure where to start, try knitting or crocheting one in any pattern using soft, pure white baby yarn -- maybe adding a small satin bow in pink or blue in one corner.
On the pattern link above, you'll also find some instructions for tiny pouches. The hospitals use these for very early losses, since these babies are often too small and fragile to be dressed at all. On this one, I put an extra layer of batting at the top to form a tiny pillow. The plaid lining makes for a sweet look. This one is made out of cotton quilt fabric, but I always think a flannel lining is an especially nice touch, too.
I hope these couple of posts have helped inspire some ideas for you as you consider crafting projects that really fill a need. Your local hospital volunteer department will be able to give you some more specific ideas, too. I've found that, while many hospitals have lots of volunteers to make baby hats and other happy items, there aren't many who make bereavement items. Please consider filling this need in your area. And thanks again for all your comments.