As everyone around here probably knows, I am a great fan of swatching. I do it for fun. But when it comes to lace, especially lace where there are four or five kinds of stitches going on at once (as opposed to a simple shawl that uses the same lace pattern through the whole thing), well, it doesn't work. There's too much variation in blocking, stretch of different stitch patterns, stretch of the same patterns used in different places, blah blah blah.
Best thing to do? Take notes and knit a lot of lace medallions. Lucky for you I've already done that.
Let's take a walk down memory lane.
Back in the day, I was a crazy-ass documenter of everything, and after I knit a doily or two, I thought "I should be writing this down." so I got a notebook, and did. It's this one. In case you can't read my writing, the date on the front cover is March 18, 1986 (in cursive! I never re-learned how to write in cursive after I broke my hand. Truly a historical document). For the first ten, fifteen years I knit, I documented all my lace projects in here, xeroxing the patterns and gluing them in and writing notes to myself all over them. See?
I even kept track of how long some of them took to knit, in case of emergency gift knitting.
Nowadays my pattern collection has outstripped my little book, but I still use it to go back and remember what I knit for whom, in what color. (Yes, I wrote all that down too.) And it works well as a general basis for gauge, if you write down how big all your doilies and shawls are when you finish them.
The last thing I knit in my Early Doily Phase was this:
As you can see, I wrote down that on size two needles, it blocked to forty inches in diameter. Knowing that, I have cast on this brown shawl I am knitting with size threes; the number of rounds in both doilies is the same (or near enough), so with a slightly larger size needle, I should get a slightly larger medallion. Which is what I want. Added to that, animal fibers stretch more on blocking than cotton - meaning the new shawl will be larger. And openwork fabrics with lots of eyelets tend to stretch more than solidly knit fabrics - meaning the new shawl will be larger than the old 'swatch' doily.
These days, with the joy of Ravelry, you can look on line and see approximately what size you'll get with a certain pattern and certain needles. But here are some general guidelines:
-It's a circle or square. Even if it's for a shawl, an inch either way isn't going to make a difference when it comes to fit, so relax a little, already.
-Save weird shapes like hexagons and ovals for a later project if this is your first try.
-Animal fibers relax more and stretch and drape more than plant fibers.
-Openwork fabric stretches more and becomes larger than an equal number of stitches knit in stockinette. Any kind of crossed-stitch pattern sucks in, is very firm, and will eat yarn (meaning use up a lot of yarn, FAST).
-On size two needles, you will use up six hundred yards of thread at approximately round 110, and a thousand yards of thread at approximately round 145. (You can obviously adjust up or down from this figure.)
-To state the obvious, each round gets longer as you knit outward. At about round 110, you will have about a thousand stitches on the needle. Each round uses progressively more time and thread to knit.
-Again for size two needles, a 45 round doily will block to twelve inches in diameter. A 90 round doily will block to twenty-four inches in diameter. A 160 round doily will block to three and a half feet in diameter. Approximately.
-On quad-zero needles, a 90 round doily will block to twelve inches.
All this is approximate, but it gives you SOME idea what will happen, when you cast on. So there you are. Toddle off and cast on something fun, kids. And those of you who knit socks, these are the same damn thing. Really. Just with more holes on purpose, and they lay flat when you're done.
(There's a great free doily pattern here that would make a fine test subject; it is both charted and written out. And a great wad of knitted doily patterns for free, here, originally from American Thread Company. Think free Lion-Brand patterns to sell yarn, but older and classier. And no fun fur.)