The frame size is not an exact science but it does have to be Right. Machta mentioned that Most men ride a larger frame than necessary. Most women have shorter torso's than men and this is why Women Specific Frames are now made. Same height or Standover on the two frames- just a difference in top tube length.

Two things I look at on bike fit and the first one is Standover height. If I do not have good clearance on a road bike between the crotch and the top tube- then it is too big. Then the feel of the bike once I am in riding position. I know I can change bar stems and adjust fore and aft on the saddle, but if it is not nearly comfortable in standard form- then the bike does not fit and look to a different Size or different Model.

Once the frame size is sorted then you can think about changing parts to "Finetune" but I bought a road bike this year- I am a mountain biker. Only parts I have changed are the Bar stem to one 20mm longer and the pedals. This is probably due to 15 years of MTB where a more upright stance is used. It fitted Correctly in the shop and I will not be buying a new one that fits better next year. Picvture attched but this is a Compact geometry frame so looks wierd- and was before changing the pedals and stem but this bike fits. This is the first bike that has the Saddle central on the rails and not fully forward or fully back so is probably the first frame that is my correct size aswell.