I'm pretty sure there was a description of it in
Eclipse. When Alice shows it to Bella, Bella remarks that the wedding theme (and dress) was a 1918 theme, which was more for Edward's benefit than anyone else's.
Edit: Ugh, there is no description, but Bella mentions it being similar to Anne of Green Gables.
Also, while searching for 1918 wedding dress pictures, I got lucky and hit
this site.