Your knowledge on Yellow's are way off.
Males 7-9 feet
Females 9-12 feet.
If you do not know for sure, then please keep your input to yourself.
If you're going to get a Burmese or a Yellow, I would highly recommending you getting one as a baby, that way you feel more comfortable with the snake as you and the snake get older, as well as while the snake grows.
The only way I would really get a larger snake, is it would have to be from a highly valued breeder, and it would have to be in person, so you can see first hand the temper of the snake as well as the condition.
If you are looking to get a Yellow, please place the peoples information that have dealings with them, and currently own them on top of people that "just hear".