First, how did women find shawl patterns? Women's magazines and needlework books had plenty on offer, but many were poorly edited. The shawl on the right, for example, came out of Mrs. Beeton's needlework book. If you try to follow the directions, you'll realize that the lace pattern on the back is not the same as the illustration. Worse, there is some sort of huge error in row two of the border lace, making it impossible to complete.
This particular shawl pattern appears in several period sources, each copying or compounding the errors of some ur-shawl.
I am certainly all for going to period sources, but do expect some degree of frustration.
Another option knitters used was to look at other women's shawls and try to copy them or even one-up them. There's a great quote from a period diary about a shawl craze in one southern town where local women vied to create the most vivid, intricate borders.
So a second option is to look at period-CDVs for inspiration.
The simplest option might be to design your own within period parameters.
Read enough period patterns and you'll start to get a feel for period yarn sizes. No one will yell at you for lace weight, fingering weight, or sport weight. If you want to go heavier, be prepared for an argument. I'm not saying no, but I am saying that some folks have made up their mind on this matter.
It should go without saying to use natural fibers for period knitting, not blends. Some fibers to avoid or be prepared to defend are silk, mohair, or angora. Wool is the most defensible, and since shawls were designed for warmth, cotton would be an unlikely choice. Wool. Definitely wool. Wool is safe.
Needle sizes should be proportionate to your yarn, allowing it to drape when knitted. Size 5s are a good bet. If you're knitting at home, go ahead and use those circular needles or blue aluminum, but at an event go for an appropriate needle: wood, bone, steel. For long projects, they used many sets of double pointed needles. Once a shawl gets to a certain size, I just tend to work on it at home.
The basic shape to aim for is a triangle. I have yet to see a period shawl that is circular. Again, I'm not saying there weren't any, but I'd want to see a period photo before spending dozens of hours knitting something incorrect.
The most basic shawl would be like the one above (which is shown folded in half). It's done in garter stitch (all knit). Cast on 5 stitches. Increase 4 times every other row--once at each edge & once on either side of your center stitch. I like a yarn-over increase as it makes a pretty lace effect. Stripe as you will. Consider slipping the first stitch on each row for a tidy edge and to make fringe easier.
Why start with only 5 stitches and work your way wider? At first, it doesn't seem to make sense, for each row will get progressively longer, and potentially more tedious. I know; I knitted this shawl myself. By the end, I never wanted to see garter stitch again.
The advantage, however, is that you don't have to worry so much about gauge or running out of yarn. You simply knit until it's big enough.
That's pretty much it. Don't forget to send photos.