What size curling wand...

[Tinks15]

for bobbed hair to create beachy waves?

Not sure but if you have a sainsbury near by they have a big selection of curlers/straighteners reduced to clear

Have you got thin straighteners? I find they make waves better than a wand.

Try a conical wand. Babyliss does some good ones for around 25 quid. They're mostly all a similar size.