I think there is a bit of confusion here between the "day plaid" and the full plaid. The day plaid is illustrated in Tobus' first two pictures and is, essentially, a tartan blanket folded and carried over the left shoulder. The full plaid, as worn by Euan MacPherson of Glentruim, is the plaid that is four yards by one and a half and worn wrapped around the body (those are the dimensions for a full plaid given by Adam Frank, by the way).

If you'd like to make a day plaid, Chas' recommendation on size seems about right; if a full plaid, 12' x 4.5' seems to be about the norm.