Hi Darren,
A dilemma most of us men face at one stage or another!
The good news is that this type of classic engagement ring can be changed in size after you propose.
As a rough guide though....the most common sizes in the UK are M & N, smaller women will have fingers of J, K or L and slightly larger are O, P and Q. Having said that we sell rings from as small as F to as large as Z!.
The best thing to do is take a smaller, medium or larger size depending on whereshe is on the scale. You won't be too far out and then we can amend to fit afterwards.
If you need more help ask away by email or feel free to call us on 0845 084 1200.