How to estimate amount of all purpose flour for roux?
Typically, you go for 1 tbsp flour, 1 tbsp butter for each cup of liquid. 
You may have to adjust slightly, but that's a good starting point.

After the quantity of roux, the next factor to thickening strength is cooking time.  The longer you cook the roux, and consequently, how dark it gets, the less thick the final result will be for a given quantity of roux and liquid.

For a white sauce (i.e. Bechamel and Bechamel-based sauces) you want the roux to stay more or less white.
For a Gumbo, you cook it until it's light brown (sometimes called red).