ok my thruppence worth
i have made several beds
sides from 6x1" par[planed] sides 150mm longer than the matress
the sides overhang the ends by 44mm this allows the legs /head foot boards to be secured on 2 edges giving a very strong fixing
screw and glue a bit off 2x1 par along the inside bottom edge off the sides and end
use 4x1" for the slats chamfer top edges to stop dammage to the matress space 60 to 80mm apart
for example a 6ft3" matress will require 12 slats one at zero one at 6ft3" and 10 inbetween with a 70mm gap between them
you will also need a bit off 2x1" running down the centre scrwed to each slat to give extra support
the length off the legs should be governed by the storage boxes or whatever you plan to put underneath the bed
buy your timber from a timber yard or wood yard the wood from sheds[bxq wicks ect is carp bent full off knots it also only comes in short lengths causing dreadfull wastage for example a 3m length off wood will only give you one 5 ft slat with 4ft10" waste where as a 3.3 or 4.8 length will give 2 or 3 lengths with a few inches off waste
if you do completly egnore all my advice do NOT egnore the slats information as 2x1 sawn timber is exteriour poor quality