Hi,
150 x 30 mm should suit a low raised bed.
I am with Hawfinch on this one though, slugs love decaying wood. There have been a few articles recently that suggest using the 16 inch concrete roof tiles half buried as an edging. Haven’t tried these yet to know if they cause slug problems too but they can be found very cheaply or free on the usual websites.
If you go with the timber try Danish oil as a preservative, works well and is food safe.
HTH
Tristan