These firm flexible plastic grid sheets are an essential part of any flat bottomed bag.  They are easily cut to fit your project and are available in 3 sizes.
 
Small - 13 7/16" x 11" (343mm x 280mm)
Medium - 18" x 12" (457mm x 305mm)
Large - 22 3/8" x 13 7/16" (567mm x 343mm)
Emma Tote - 11cm x 28cm - enough to make one tote bag from Emma's Tote Bag Stitch-a-long, details of which can be found here.  Please note this size is only suitable for the stitch-a-long pattern.
 
This product may be rolled for shipping.  Simply place a heavy object on the top to flatten it back out.