Featured Ring advice please

I know this isn’t really antiques related but I respect your knowledge.
I have just bought this ring as an 18th birthday present for my niece- (not one of my £1 buys). I looked high and low for one that I liked and I love this as it’s delicate and beautiful. The problem is that I think it’s smaller than the size N it’s meant to be . By my measurements it’s a L L1/2 as it’s 16.36mm diameter.
OH is going to buy a ring sizer on his way home to be sure but do I have the option of getting it resized or is that just not worth the hassle and expense (or would it ruin its strength/ value ) and I should just send it back and search for another one?
Gutted as I love it, the photo doesn’t do it justice.
It’s 9ct with little diamonds and ruby/garnet with a full hallmark. Not huge money (c£65 with the postage) and presume the cost of resizing it may be more than it’s worth.
Advice welcome Thankyou

 

I think I'd return it if you can. Resizing would cost as much as the ring.

 

I thought so

 

I do happen to have this, if you're interested.

 

Lol, love an opportunist . Is that opal in the middle?. Ruby is her birthstone so that was the important bit. What size is it?. Can't read the hallmark- what does it say?

 

It's gold washed silver I think, rather than gold, there's no content mark. Looks like an opal to me. Date letter is for 1976, so it's vintage, but in decent condition. My ring size used to be M but these days I wear N. It's very slightly loose on my left hand, fine on the right.

 

Thankyou. I am looking for a gold one but thanks for the offer .

 

Someone call a Moderator for trying to sell on the non-selling forum....
Oh, you are one.
In there like a pro @daveydempsey
That's gorgeous but I suspect somewhat above my price range.