Garage Conversion

[glenann]

As per the title, I have an opportunity to build a room within a room in my brick built garage. As I said the walls are single brick sat on a concrete pad. I'm trying to keep it as simple as possible, so it'll be a floating floor and then 3x2 stud wall with rock wool insulation throughout. I'm getting a joiner to do the stud work for me.
So there are two points really, first does anyone have any useful tips before I start? And secondly and probably more importantly, what size should I make it? Two of the dimensions are fixed. The width will be 9ft, maybe a couple of inches more. The height will be about 7ft, possible reducing toward the back of the garage. So what length should I make it? I can go to about 16ft. Should I make it as long as I can, or does the golden ratio come into effect? I have no clue about these things, so any input would be most welcome. I use Harbeth P3ESR's at the moment.

Thanks for any input...