What is the optimum solar cell sample size that I should select while testing it with PASAN SunSim 3B Sun Simulator?

I have to suggest sample sizes in order to generate some I-V curves using PASAN SunSim 3B Sun Simulator. The issues with the size and other things was about difference in distance to the light source, and the fact that the light source is not collimated and we can't collimate it, so we need to workout the tolerance.
The distance between the external lamp and the sample is about 7 meters.

External Lamp consists of 4 lamps connected to a single square shaped holder.
The dimensions specs of the external lamp as given in the attached images.

What if the sample size is smaller than the light source?
What is the optimum solar cell sample size that I need to select?
What parameters will be effects if the sample size is not optimized?
"Please Keep In Mind That The Sample Size Should Be As Small As Possible".

The attached images with help you in getting an idea, how the sun simulator looks like and what are the dimensions of external lamp



Picture of What is the optimum solar cell sample size that I should select while testing it with PASAN SunSim 3B Sun Simulator?
Pasan External Lamp.jpg
External Lamp Specs.jpg