Mountain Bike Frame Sizes. Part of the series: Bicycle Repair. When deciding on a mountain bike frame size, it's important to have an understanding and knowledge of one's inseam and stand-over height measurements. Get the mountain bike that's the right fit with help from an experienced bicycle mechanic in this free repair video on mountain bike frame sizes. Read more: http://www.ehow.com/video_5773300_mountain-bike-frame-sizes.html