Done this a number of times before ( its my day job)..... We always
use 6x2 at 600mm centres unless its a really short span of say less
than 1.5m where you might get away with 4x2.

CLS equiv may be an alternative to consider 148 x 38

B&Q had a great deal on this ... cheaper than local timber yard .... great
for this type of job as it's all gauged and planed.