During the reionization era, ionizing photons from early galaxies carve out large ionized bubbles in the intergalactic medium, which grow and eventually fill the Universe by z~6. Over the next decade, astronomers, using instruments such as the Hydrogen Epoch of Reionization Array (HERA), hope to image the last remaining neutral regions using the 21-cm line. Unfortunately, although there are many models describing how ionized bubbles grow during reionization, there is little understanding of how the sizes of neutral regions depend on astrophysical parameters, even though they are closely related problems. We measure the sizes of these regions in semi-numerical simulations in order to study how they depend on various astrophysical parameters. Here, we develop a Gaussian blur method to probe the sizes of these neutral regions. We then compare our results to other publicly available methods like the mean-free-path (ray casting) approach and friends-of-friends (percolation) approach, and find an agreement within an order of magnitude. Using these methods, we find that the mean free path of ionizing photons and the number of photons produced per galaxy are the most important parameters for determining the neutral region sizes, after controlling for the average ionized fraction. We also study how the size distribution depends upon the model for absorption in the intergalactic medium, demonstrating that an accurate treatment is essential for interpreting observations.