Hi Bea,
Sorry - no existing posts about sizes to make doll jewelry.
I suggest that for doll bracelets and necklaces, you use
Stretch Magic cord, rather than
standard beading cord or beading cable. This is because knotting accurately with non-stretch cord on a 7.5" bracelet is already difficult (you often get get a tiny bit of slack). But that 1-2mm of slack that might be acceptable on a 7.5" bracelet or 18" necklace is going to look awful on a 1/2" bracelet or 2" necklace. And, the 2mm crimp bead used when finishing beading cable for human-sized jewelry, will be over-sized for doll jewelry.
The Stretch Magic should also work nicely for doll headpieces. It knots very nicely.
Small sparkly beads such as
Charlotte-cut seed beads or
3mm Czech firepolish beads should work nicely for doll jewelry.