its simple, just ask her :P she should know her size or drop in a jewelry shop and ask! if you really feel awkward, go to any jewelry section in a mall, try out random ring sizes, check the size of the ring that fits you on the tag... lol

About the dress, i think it varies in religion to religion, on what kind of culture you follow.. In my culture, the bride buys her own wedding dress, but swayamber ma, the guys family is supposed to give her a set of clothing, that includes jewelry too! Check with your relatives or your family on it!