Here's a related factoid set, just for fun, but also because you may find an old plate holder that was sized for an expected standard.
From the 1960 edition of The Focal Encyclopedia of Photography p 1142.
Plate Sizes and Thicknesses:
2-1/2x3-1/2 (6x9 cm) => 0.04 - 0.05 in ( ~1.0 mm)
4x6 in (10x15 cm) => 0.05 in (~1.3 mm)
8x10 in (20x25 cm) => 0.06 in (~1.5 mm)
12x15 in (30x38 cm) => 0.08 in (~2.0 mm)
> 24 x 30 inches => 0.13 inches! Uff dah, my aching back.