If that's the case, then what you're looking for is in the Brush settings panel (you can access it by clicking on the rectangular icon next to the brush size setting in standard interface.
If you want constant brush size, deactivate the "Shape dynamics" option, and as for stroke opacity there's a button next to the brush size setting that displays a check board motive and a pen. Pressing it will activate "pressure for opacity", which seems to be what you're looking for.
You'll probably want to take an extended look at Shape dynamics to learn more about how to configure pressure. There are some cool features you might need later on in here.