Depends on many factors. Many MJ plants show preflowers after about 6 weeks so they are easy to sex, while others have to be in 12/12 for a week or 2 before they show. Still others will autoflower and go into flowering even under 24 hours of light per day. It really hasn't got much to do with size more to do with time and maturity.
There may very well be strains that you can tell the sex after 7 nodes, but in general this is not true.