Why would you get a tree based on diameter. Height is the real limiting factor when getting one when going full sized as that implies you have space for it. Quick search says that its not unusual for a 6 foot tall tree to be 6 foot wide. So maybe go with 1 to 1 width to height at most. Though I think you will most likely see something along the lines of 1:2 which means that 6-7 foot tall tree might end up being 12-14 feet tall.
Note that 2:3 is more aesthetically pleasing than a tree what is 1:2 but that still means your tree could end up being over 10 feet tall.