My understanding from that was they were measuring it to make and test a computer model to simulate the shield in 0 g, not to get exact measurements to "it to the right size". Now its in space, they know how it performs and how accurate the model is. I definitely could be wrong.
Measuring the sun shield alone was a 5+ year long project.
[1] https://youtu.be/Pu97IiO_yDI