Earthen masonry generally is brittle, weak, and poor in damage resilience. There is historical evidence that natural fibers, such as straw and horsehair, have been used to reinforce earthen masonry to prevent desiccation cracks and improve tensile strength. However, fibers also have been known to affect mechanical properties negatively, such as with compressive strength (an important quality control parameter for load-bearing masonry) by creating voids and lowering density. This paper reports on findings of a study directed at investigating the feasibility of avoiding such problems in compressed and stabilized earth blocks through optimizing the fiber length when using soil from Newberry, FL. Standard polypropylene fibers were selected for the study. The two lengths of fibers studied were 54 mm and 27 mm. The test results showed a general improvement in compressive strength of the fiber reinforced matrices compared to the unreinforced ones. Although an improvement in modulus of rupture (MOR) was observed for matrices reinforced with 54 mm fibers, results varied for the other fiber-reinforced matrices. An improvement in post-initial crack behavior was observed for all fiber-reinforced matrices compared to the unreinforced ones. The 54 mm fibers yielded the best results based on the influence on MOR, compressive strength, and deformability compared with the other matrices.