The aim of this study was to characterize ankle stability of total talar prosthesis (TTP) and to determine the effect of implant sizes on stability as well as the resistance to TTP dislocation. Twelve below-knee cadaveric specimens were divided into two groups. Group 1 received a size matched implant and Group 2 received downsized implant by 5%. The stability assessment under fluoroscopy was performed for each cadaver in its native state. Following TTP insertion process, each then underwent evaluation of the TTP ankle stability. The stability of pre- and post-TTP was compared. (1) Anterior drawer distance. (2) Talar tilt angle under varus and valgus stress. (3) Subtalar tilt angle under varus stress was measured. Finally, the dislocation test was performed using the aforementioned testing conditions, then the stress force was slowly increased from 0 to 350 N, during which time it was observed on fluoroscopy all the time. Compared to pre TTP ankles, varus and anterior drawer stress showed significant instability (p < 0.001–0.031). Only anterior drawer stress in smaller sized implants showed significant instability when compared to identical sized implants (p = 0.008). No dislocation was seen under varus, valgus, and subtalar stress. However, anterior dislocation was observed in all cases of smaller size implant group (p = 0.045). TTP implant was stable under valgus and subtalar stress. However, clinicians should pay attention to anterior instability. Notably, downsized implants should be considered carefully to minimize the chance of anterior dislocation.