Determining which factors influence idiopathic macular hole (MH) size is important because it is a major prognostic indicator of treatment success. Foveal pit morphology is highly symmetrical within individuals and may influence MH size. Using a series of patients with unilateral MHs, we examined the foveal floor size of the fellow eye to evaluate its relationship with MH size and post-operative outcomes. A retrospective observational study Participants 241 participants with a unilateral MH treated with surgery and a fellow eye with no ocular pathology. Spectral domain ocular coherence tomography (SD-OCT) imaged both eyes at the time of surgery. Minimum linear diameter (MLD) and base diameter (BD) defined MH size. Foveal floor width (FFW) and minimal foveal thickness (MFT) defined foveal pit morphology of the fellow eye. Baseline characteristics, SD-OCT measurements and pre-operative variables were compared to determine their relationship with MH size and post-operative visual acuity in logMAR units (Va). FFW was correlated with MLD (r = 0.36; p=<0.001) and BD (r=0.30; p=<0.001) but not post-operative Va. MLD correlated with pre-operative (r=0.49; p=<0.0001) and post-operative Va (r=0.54, p=<0.0001). A two-stage regression model was developed to predict post-operative Va (r = 0.28); pre-operative Va (beta = 0.36; p=0.002) explained 13% of variability and MLD (beta = 0.29; p=0.002) and MH duration (beta=0.23; p=0.004) explained a further 16%. FFW of the fellow eye in patients with a unilateral MH was significantly correlated with MH size and may explain some of the variability in MH size observed between individuals. However, FFW could not predict post-operative vision. [Abstract copyright: Copyright © 2020. Published by Elsevier Inc.]