Hi. I went through the same question 2-3 weeks ago before I made the purchase. I want to wear this necklace for daily wear that gives dainty look with enough sparkle, I want to be able to wear it by itself or layered with other dainty necklaces occasionally.
I've tried all platinum, RG, and YG, and for my skin tone I also prefer YG - platinum looks a bit dull greyish and RG looks too blend on my skin. I've also tried all standard sizes (up to 0.14 for YG and up to 0.17 for RG) and non-standard sizes like 0.21, 0.22, 0.26, 0.3 carats. I did not go beyond 0.3 because it looks way too big for everyday wear. So in my opinion, minimum is 0.14 - below 0.14 looks too tiny and in the future you would want to get bigger carat - and maximum is around 0.23 carat. Above 0.25, like I tried 0.26, already looks too big. And if you get 0.18 and above, the serial number will be engraved on the diamond and you can choose the Color and Clarity, and you can ask for Tiffany official authenticity paper. While below 0.18 carat, the Color and Clarity are not known, but Tiffany stated their diamonds always high quality, it can be anywhere between D to I color and clarity up to VS2. So I will recommend getting between 0.18 to 0.23 carat, just choose the biggest one you can get within this range. If you have 2 options with similar price between smaller size with better Color/Clarity vs bigger size with lower Color/Clarity, I will recommend getting the bigger size over the Color and Clarity because it is still quite small diamond and it does not really make much difference in human eyes for Color/Clarity as long as they are in the top range, which Tiffany standard is.
For me, at the end I bought 0.22 carat in I and VS1/VS2. I had another option for 0.21 ct in G and VVS2 with exactly the same price, but honestly you can't really tell the difference between those 2 unless very close inspection, but you will notice the size difference, so I chose the bigger size.
Hope this will help you in purchasing this necklace.