Need help with hybrid bike sizing for my wife. She's 5'5" (168cm). Her standover height is 31". I did search on-line and came up with 17" based on her height/standover. Now, on-line retailer recommended 15" bike. She knows how to ride and it is not a novice that needs to be close to the ground just in case :-) 15" to me looks like it could be too small.