The "box" does not have to be square...
...it can be any shape, circular, ellipse, rectangular, or multi-sided. I requested (and was granted) a 6-sided shape, 40 miles x 80 miles, for my Phase I flight test area.
Your physical location, proximity to busy airspace, city size, etc will weigh in with what you will be granted. When you submit your paperwork to the FSDO, or DAR, include a sectional with your proposed flight test area well marked. You may just get what you ask for...