The maximum size depends on both the patch type and the shape, and some options like a merrowed border & Velcro are often not available at unusual sizes. Here’s a quick guide:
The maximum size for Rectangles is 30cm x 33cm.
The maximum size for Squares is 33cm.
The maximum size for Circles is 33cm.
The maximum size for Unusual Shapes is currently 30cm x 33cm.
Merrowed Borders are available at most sizes for Rectangles & Squares, but is limited to between 6cm to 10cm on Circular patches. Merrowed is not available for unusual shapes.
Velcro Backing is limited to 10cm on most patches, although 30cm x 10cm, or 15cm x 10cm etc, is perfectly ok, provided one of your patch dimensions is under 10cm.