Your raw post count (i.e. the total numbers of posts you have made in the entire forum that is found beneath your avatar) is what must have reached a total of 30 to be awarded a place on the map. Thus far there is no strict rule as to size limitations, but it is expected that you will request a reasonable area given your population. Oz will determine if he thinks your request is suitable. He is very fair about the map, so I wouldn't worry about it.
For the record, I do believe a number of nations have either sent or attached sample maps upon which Oz has based their country's borders.