"I mean i know how to build from a schematic but how do you know that lets say for example R5 is too much or C2 is too low?"
This is a loaded question and really depends on some things.
If you are studying a schematic for a proven circuit or a commercial device that is functional, you probably wouldn't be too concerned with whether or not a component is too high or too low...in terms of understanding the design. A very, very, experienced and knowledgeable designer might be able to critique a design and change things he feels are inadequate, and even an intermediate hobbyist might fine tune a design he finds on the internet or in a book, but most of the time we wouldn't worry ourselves with those details.
Now, if you are designing your own circuit, and you are unsure about the appropriate size or type component for an application, technical books and datasheets are the roadmap you would use, along with your fundamental knowledge, intuition, and sometimes a little trial and error. It can take years to develop the foundation of knowledge required for circuit design. I myself have taken on only a few projects of my own and have not ever designed anything of any complexity that I would publish LOL. I am happy with building proven projects and giving them my own little signature here and there.
Lastly, if you are interested in troubleshooting and repair, as far as reading schematics goes, electronics is like any other repair profession...for instance auto mechanics. You follow logical procedures, analyze functional blocks and systematically test them. Draw info from documentation and specs, and most importantly you learn how to use your test equipment properly to verify.
It isn't always imperative that you know every component and have all the values precisely calculated. But when you study a schematic, you will get gut feelings on the nature of the fault and what components might be suspect. You will then familiarize yourself with the function of and specific values, outputs, waveforms, and voltages that these components should produce. You will then proceed to test them under operating conditions. Most of the time, a failed component will make itself known with physical signs of damage, but studying a schematic will give you hints has to other components that may have been involved. For instance, if a voltage regulator in a circuit is fried, I would be interested in looking at the schematic and seeing the various loads that the regulator supplies. I would then concern myself with load devices that are high current draws or prone to shorting or overheating. Before replacing the regulator and testing it live, I might confirm the other components are properly working or even replace them if I feel they might be intermittent.
These are just a few examples of how one learns to read schematics first by learning the basics and building upon them with experience.