Woodbridge Antiques has dated this piece to the 1930s.
The ring size is UK 0 / USA 7.5.

The maker's mark is also clearly visible for HG & S (Henry Griffith & Son).
A very classic ring in lovely antique condition.
Hallmarks and maker's marks: Birmingham 1923 by Henry Griffith & Son
Measurements:
Ring size: UK 0 USA 7.5
Width across band at top: 8.4 mm approx.
Width of band at base: 2.9 mm approx.
Weight: 2.9 grams approx.
Woodbridge Antiques has dated this piece to the 1930s.
Specialists in quality antique silver, fine jewellery & accessories
You can view the stock and full dealer information on the Woodbridge Antiques Profile Page