If you look at Old Irish & Highland Dress by H. F. McClintock, the brat in Ireland may have well taken many forms over the courst of the centuries. All a brat is, really, is a wrap or a mantle. So, as fashions change and the centuries roll on, one would naturally expect it to take some different forms. Some were square, rectangular, semi-circular, some had a hood, some didn't.

The brat that would later develop into the feilidh-mhor or belted plaid (in Scotland, mind you, not in Ireland), was more or less a rectangular length of woolen cloth, somewhat smaller than the feilidh-mhor as we know it. I'd say something 50" to 60" wide and 2 yards long, more or less, would suffice.

Aye,
Matt