Measure whatever they usually wear

e.g. bangle, necklace, bracelet, earrings.

Rings can be more tricky to measure surrepticiously : you could borrow a ring & take it to a jeweller to measure, or you could send it to me recorded delivery & I could measure it.

Remember that different fingers will have different sizes.

I can send you a paper ring sizer for them to try on.

Because if the size is wrong a stone would have to be removed from a ring to resize it, I can send the ring before a stone is set to be sure it is comfortable.