Gowns sizes are different than sizes for clothing such as slacks, jeans and tops. And, designer gowns can vary in size depending on the country where they have been made. If you find a gown online that you like, see if you can locate one in a shop to try on before buying. If not, then check out the sizing information on the website and measure carefully. Also be sure to read the online shops return or exchange policies.
Hemming depends on the dress. If the hemline has a trim or other embellishment that would be more difficult to alter. Consult with a local seamstress before buying a dress to be sure. Top Wedding Questions Forum Moderator - "Write your sorrows in the sand, your blessings in stone".