Hi frostgfx. Age, weight, sex if determined, and a photo should give the basics. Shed, feeding, defecation, and behavior notes complete the picture. While length is interesting, it just isn't that important. Proportion, as in the loaf of bread shape in imagined crossection, is more important than length.
I was able to see my snake before purchase, but because I got him at a pet store I don't even know his exact age or his sex. I also didn't get records other than a casual "was fed a pinky once a week" on Thursdays. So even without seeing the length, but by going to a breeder, you will have a lot more info than I have.
If you do have trouble correlating the very good pictures in the thread to actual snake size, then go to one of the big box pet stores and just look at the baby corns. It will give you a good idea of just how little the babies are.
And they grow fast. I've had mine for just under 6 weeks. He has had a shed and and he looks a lot longer. Whatever length you get as a baby, I guarantee will be longer soon.