

If you constructed a circle with the radius of the universe, then measured its circumference and radius measurement accuracy would easily be able to tell the difference between a real circle and a mathematical circle. That is because neither the perimeter of circle will nor the diameter of the circle will be through in empty space. They will be near enough to matter to measure detectable deflections.
I mean, geodetic interferometers already exist and can measure very small deviations. Give them arms the length of the observable universe and they will increase in accuracy, not decrease in accuracy.