There is no "standard size" for a typical fly plaid, but they are almost always square, or close to being square. 36" x 36" is a common size. I've made them for people by request as large as 54" x 54" before. Sometimes when I'm dealing with single width cloth I've made them 36" x 28" because that was how wide the fabric was. Because of the way the plaid hangs when worn, no one really notices that the plaid isn't quite square, anyway.

I've heard it said that the bottom tip of the plaid should not fall lower than the bottom of the wearer's kilt, which would, of course, require everyone's fly plaid being made custom for them to the right size. But most commercial firms these days don't offer that option and what you get is a standard size.

However, all of that being said, I really don't like the look of the so-called "mini-fly plaid." The one mentioned here was 15" by 54". That's only slightly wider than a typical tartan scarf, which (depending on the manufacturer) typically measures 54" long by 10" to 12" wide.

I've seen men wearing tartan scarves or even lady's sashes (same width, but longer, typically 72" to 90") on their shoulder as a fly plaid, either out of ignorance (they think this is where it ought to be worn) or because they were looking for a cheap alternative to a full fly plaid. It doesn't look good. It looks like you don't know what you are wearing. This is why I never have understood why some companies have introduced the "mini-fly plaid" to the market. It's just a glorified scarf.

I've seen it modelled as casual wear in some catalogs, but I still don't buy it. It just doesn't look right no matter how you wear it (unless you want to wear it as a scarf!).