Jeff,
MANY kudos for educating yourself and putting such effort into a gift for your wife! I'm also a beginner, so I'm going to defer on the Akoyo vs Freshadama question to our more knowledgeable folks, except to say that if she has her heart absolutely set on Akoyos then it may be best to start there. Sometimes us women folk aren't rational about not getting what we expected, regardless of the fact that what we're getting might be
better than what we expected.
As for size, I'll freely regurgitate many comments I've read from others in threads here at P-G: Many posters have mentioned being 5'-5'6" and still wearing larger pearls very happily, and they seem to indicate that 'too big' is often an unfounded fear in the realm of sizes folks here talk about (maybe... 7-13mm?). My personal fav (8-9) was determined based on my desire to compromise quality and quantity within my spending allotment.
As for length, think about what she wears most often. If she wears a lot of turtlenecks, mock tees, or high-neck items, 18" might be more versatile to wear over the shirts. If she wears a lot of button-down shirts or v-necks, etc, 16" would sit above the shirts more often. If she wears a lot of low-cut shirts, I imagine either would be great!