I want to dry line a room in the garage, whats the best size timber to get for the battens, and what size does the plasterboard come in, is it 8 x 4 ?

Just so I can do some rough pricing :)

Plasterboard comes in 4x3, 6x3 and 8x4 and 3/8" and 5/8" thick

8x4's are a bit difficult on your own to manouvere 6x3's are ok though

I've not used battens before and used dry wall adhesive

I would imagine that 25 x 38mm batten should be ok and you can nail or screw them

dry linning has massive variations if its a garage make sure you insulate well you can buy 60mm thermoline 8x4 sheets for around £30 each sound a lot but excellent for insulating. normal 8x4 1/2 " pay no more than 3.80 a sheet i pay £3.20 but i buy a lot. Board Adhesive £4.00 bag skim £2.90 bag

if your not sure on different things pm me will happy to help.

if your doing it in the garage why not just buy 50mm metal stud !! it will be quicker easier and better to install plus it will be easier to fit rockwool behind it to insulate :D as for boards why not add the soundbloc boards in 2400x900x15mm they will be easier to handle as compared with the 1200mm wide boards and will give all needed building regs etc if you are pushed for it :)