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Having your bike correctly fitted is extremely important for comfort, performance, and efficiency on your bike. Bike fit is more than just having the right size frame. It means having your body placed in a position which it can support without discomfort or injury.
You can adjust your bike fit yourself or have it done professionally. You can go quite a long ways doing a DIY bike fit, but a professional fitter has tools and biomechanical background which can dial your fit in to perfection.
Here are some of my notes on adjusting various parts of your bike on your own.
The following is based on SPD-SL cleats, but the theory should apply to all cleat systems.
If you are using clipless pedals, cleat placement becomes the absolute most important part of a bike fit to get correct, as the effect of incorrectly adjusted cleats will propagate all the way up through every other aspect of your fit, starting with destroying your knees.
These instructions describe the method described in a video by leonardmlee on YouTube.
How to Fit An SPD SL Cleat To A Cycling Shoe
The goal is to adjust your cleats such that the ball of your foot is placed above the pedal spindle, and such that your foot is able to remain in its natural orientation when attached to the pedal. The following instructions should be followed separately for each foot, since it is likely that your feet are not exactly the same.
First, you need to figure out how far forward or back the cleat should be on your shoe. Measure the distance from your heel to the ball of your foot. You should stand up against a wall and use a rigid ruler to do this, making a mark on the ruler where the ball is.
Then, translate this measurement to the shoe and make a mark. Placing a white piece of tape and the exact location on that tape with a permanent marker is a good way to do this without leaving a permanent mark on the shoe.
SPD-SL cleats have a dot in the center of them, indicating where the spindle will end up relative to the cleat. You should align this dot with the mark you made on the shoe.
Next, you need to figure out how the cleat should be rotated. Starting with the cleat centered, make an indicator on the forwardmost tip of the cleat and on the shoe right in front of it (using tape as before), so that you can visualize the angle it has been rotated to.
Then, sit on a high enough ledge such that your legs dangle freely. If you look at your feet, you will observe that they are rotated either inwards or outwards. Hold your shoes upside down and translate this rotation to your cleats. When the shoe is right side up, the angle of the cleat should be exactly opposite of your foot, so that your foot can stay in its natural orientation when attached to the straight pedal.
The saddle fore and aft position is one of the sketchier settings to change. The aspect affected by saddle setback is your pedalling performance. Saddle setback SHOULD NOT be changed in order to adjust for an incorrect reach. The appropriate saddle setback is affected by multiple other cockpit adjustments, including reach and saddle-to-bar drop, so should not be played with until most other settings are dialed in.
If you are riding the wrong size bike, it is not the end of the world, as long as you are not dramatically off the mark. As long as your standover height is acceptable, most other settings can be fixed, although it can be pricey as many settings require swapping parts.
The number one setting that will be hard to get right riding the wrong size is reach. While things like bar width and crank length may only have an effect on performance and a marginal effect on comfort, incorrect reach can cause terrible wrist, shoulder, and neck pains. You can adjust reach by replacing the stem with a longer or shorter stem. If you do not know what your exact reach distance should be, you can begin by finding the geometry chart of your bike and calculating from that.
For example, I ride a bike that is one size too large for me. I can see from the geometry chart that the stem that came stock on the bike I am riding is 10mm longer than the stem the come stock on the bike I should be riding, and that the top tube is 10mm longer on my bike than the bike I should be riding. So, I replaced the stem which is a total of 20mm shorter than the stock stem. This has largely solved my reach-related pain.