Building a small decked area

longwalks1

We are planing a small (4m x 3m) decked area at the bottom of the garden, to sit on an existing concrete slab that used to have a shed on it.  What size timber should I use for the frame of it?  The frame will be sat on top of 15 equally spaced out paving slabs to raise it off the ground.   Would treated 3x2” timber be enough, or should I use 4 x 2”?  It will only be used occasionally, and be supporting 2 x 2 seater chairs and a maximum of 4 people 

Browntoa

4x2 is standard