A "thin rectangle" refers to a shape that has a long and narrow appearance, with two sides being significantly longer than the other two sides. Full definition
I might make this as an appetizer for Thanksgiving made as a long thin rectangle with an added topping of either sun - dried tomatoes, kalamata olives or toasted pinenuts as a flavor contrast to the rest of the pizza. (angelaskitchen.com)
Place floured fists underneath the dough and gently pull sections of the dough out to form a very thin rectangle about 2 feet by 3 feet (60 cm x 90 cm). (whats4eats.com)
Up close, tiny, thin rectangles poked every which way. (pnnl.gov)