The impact of plate size on performance

Generally, the larger the plate, the better the performance.

10" x 12" is considered a normal size, representing about 70% of our orders.

As plate resistance is tested along the edges of the armor, the back face deformation (BFD) normally increases due to the lack of support from peripheral PE materials. 

Shooting the "crown" of the armor (highest point) is considered the true test of the armor's strength.  It represents the most arched portion of the polyethylene backing material.

Smaller sizes (especially with monolithic strike faces) will have higher BFD measurements due to the reduced amount of material involved.  

You can check this in the NIJ certified lab testing reports by looking for the "size" of the armor.