I'm confused now I thought full plot was 10 rods which equated to 30 x 10 or 300 sq yds
You are about right. Plots measured out now are probably done with a tape to your measurements.
Originally plots would have been set out with a chain measure.
5 poles long ( 5x 5.5yds = 27.5 yds) and 2 poles wide (2 x 5.5 yds = 11 yds) 27.5 x 11 = 302.5 sq yds