I actually recently realized for most competitive events like NOVA, Adepticon etc. you need to have the proper base sizes, so 25mm for all Drukhari characters. Bummer for me since I based some of my converted characters on 32mm. Everyday casual play though, and even small local tournaments, I've never had anyone hassle me about my base sizes, it is for just 2 characters.
Official GW ruling, I'm not actually certain.
The Night is Dark and Full of Terrors...