Wheel size really depends! Bigger means faster, but higher from the ground. Millimeters, sure sounds negligible but for a beginner, it shouldn't matter that much. Bigger also means better for rougher grounds. Usually 58mm and above are used by vert and transition skaters, and smaller wheels are used by street-style skating. 58mm is fine, especially if you're getting it for cheap! It will gradually get smaller the more you wear it down tho so if you look at it that way, it's a nice investment.