Do you think bike frame size is less important these days?

I didn't need the spreadsheet to confirm it. I was just glad to have a second reference.

The size of frame I should be riding is smaller than the smallest size offered by most manufacturers so it's only now I'm getting a frame made I've had the luxury of being able to spec it myself - and, wow, it's really small!

:)

Id imagine it would be small. I'm 5'7", but relatively short legs (<30"), and I tend to ride the smallest frames alot of manufacturers make.

Have you considered going for 650c wheels, instead of the standard 700c. I'd highly recommend it for someone of your height, as it will help place you over the wheels and not inbetween them (it will also aid standover). This should improve the feel and handling of the bike.