How does one determine what size bicycle the would need?

There are also companies that custom make bikes for people of smaller or larger than average stature. One issue with you is that most "adult" bikes are going to be too big for you, unless you can find a model that comes in XS, and childrens' bikes are typically heavier and made of less expensive components than you'd want, and the geometry would not be proper for an adult body.

Def. go to a real bike shop and get some advise on proper fit and manufacturers that might have woman specific models in your size.

Honestly, the best for you to do would be to go to your local bike shop and have someone fit you properly to the kind of bike you want. They will also be able to answer any other questions you might have. Good luck!

You sit on the bike with the seat at the height you want it at..

if you can touch the floor without tiptoeing than it's a good fit.

if you can barely reach floor,when you lower the seat it will probably work...if your over 5' tall you can ride a 28 in.

it depends. you stand over the bike with your feet flat on the floor about shoulder width apart. for a road bike you should have a couple inches of clearence between your crotch and the top tube, and on mountain bikes you want at least 4 inches. now realize this is a very rough fit. to tweek a bike to make it comfy may require swapping stems, handlebars, cranks to personalize it to your body and prefrences.

best and easiest way: go to your local bike shop and "get fitted"

this will ensure miles and miles of safe, efficient, comfortable riding