I guess I dont really get the point of the resistor to 100% and therefor not how to pick the value either.
"Resistance" is the resistance to current flow. Without the resistor (zero resistance or a "short circuit" when the button is pushed), you get too much current and the power supply voltage will drop to (nearly) zero.
[u]Ohm's Law[/u] describes the relationship between voltage, resistance and current. More voltage gives you more current. Less resistance gives you more current.
With zero resistance, you get infinite current... But in the real world, of course you can't get infinite current, and you can't really get to zero-Ohms either... What happens in the real world is the voltage drops, and sometimes the power supply fries or something else fries, or maybe a fuse blows, or maybe the power supply safely "current limits" to protect itself.
That will destroy your board.
Actually, it's very unlikely that you''ll "destroy" the board. You'll kill the power supply voltage for as long as the button is pushed, which means the Arduino will stop running, and it will reset (assuming it's not destroyed :) ) when power is restored. The pushbutton probably won't last too long with "short circuit current" going through it, and it' just a bad thing to do!
therefor not how to pick the value either.
Experience! ;) If the resistance is too low (or zero) too much current will flow. The resistor might overheat (even burn-up), or the power supply might get overloaded, etc. And, you'll be wasting energy...
If you short a power outlet in your house, excess current will flow, the voltage will drop and a circuit breaker will trip.
If the pull-up resistance is too high, you can get electromagnetic interference from electrical waves "in the air". There is not a lot of energy in the electromagnetic interference and you can't get much current.... So a lower resistance makes the interference "weaker".
A pull-up value of 10K is very common. But anything from 100 Ohms to 1M Ohms will probably work.