Framing size of Joists

[Rustypeg]

Hi, Im looking to build a timber frame extension at the rear of my Victorian mid-terrace house in Kent.
The size is 3m x 4m (ish, just under for building / planning issues.)
I will be placing frame on a 2 x 6 inch wall plate which in turn sits on foundation block walls ect.
My question is do i need to use 2 x 6 inch timber for all walls and floor ect or can I drop down to 2 x 4 inch timber stud?

The construction im thinking of is timber stud 2 x 4 or 2 x 6 inch, then 18mm OSB board then vapour barrier, batons, then clad in cedral weather-board cladding. internally, 75mm insulation and 12mm plasterboard.

Also any comments on I-beams for floor and roof joists v's solid timber joists ( & sizes) would be most welcome.

other info you may require. flat roof with rubber-seal epdm so should be light weight.
foundation blocks set in three rows, max. unsupported span over foundation is 2 meters.

many thanks for reading this, and any advise is greatly received.

Rusty.

[aronsearle]

4 Story timber frame flats have been built with 4" deep studs, so structurally a 2x4 will do what you want.