I expect a FQ to be at least 18 inches long and at least 20 inches wide (half of the width of fabric)
The LQS - Quilts on Plum Lane in Dade City, Florida, routinely cuts their FQs at least 19 inches long.
That's about the only place I've bought FQs recently.
The two FQs that I bought from Walmart recently were LONGER than 18 inches (yes, I did measure them correctly!)