小 + given-name character is the most common Chinese nickname template, used by friends, colleagues, and elders for someone younger. So 小瑞 = 'Little Rui', meaning a person whose name includes 瑞 (ruì, auspicious - as in 瑞雪, auspicious snow).
usage
Affectionate, informal. Pairs naturally with 老 + name for older acquaintances (老瑞 = 'Old Rui'). The opposite-formality template is 名字 + 先生/小姐.
