How do I properly size the components for an RC filter?
An simple first order passive R-C filter has a time constant of \$\tau=R\cdot C\$, and the cutoff frequency is \$\frac{1}{2\pi RC}\$. If you know your cutoff frequency, then fix R and set C or vice-versa. I think it's easier to find resistors over a broader range, so I prefer to pick an easy to source capacitance and let that drive my required resistance.

Resistors are rates for power, and you can use the equation \$\frac{V^2}{R}\$ to figure out what the required power dissipation will be for a given voltage drop and resistance is, and you should typically derate that by some amount (say 80%).

Capacitors have a voltage rating that must be respected. Capacitors also have something called an ESR or equivalent series resistance, which you typically don't need to worry about in filter applications unless your resisors are tiny, and in that case you should probably drop your capacitance by an order of magnitude and increase your resistance accordingly. 

Both resistors and capacitors have coefficients that dictate how their electrical properties vary with temperature, which may or may not matter depending on your application.