So what size anti roll bar? Is it one size fits all for the diameter?

NO! The effect of an ARB varies as the 4th power of the diameter: a 1" bar is over three times as stiff as a 3/4" bar.
This will also vary greatly by the length of the link (or arm) to the chassis: longer softens bar action.
If you're adapting an existing bar, one with splined ends makes pre-load bias easy.