As I understand it, you need to filter a 400-450 MHz signal to find a much lower frequency signal superimposed on it. The signal has 50Ω impedance, and you are looking for the slow signal to change only on the time scale of a second or so. If this is incorrect, please edit your question to be more specific.
This is a very simple problem. Since you have such a high ratio between the signal you want to block (400 MHz minimum) and the signal you want to pass (a few Hz). A simple passive filter will do very well. I'll assume that the A/D of the micro wants input signals to have 5KΩ or less. Different parts have different restrictions. This one would suite many. You can adjust the values accordingly if you need different.
I would probably use 2KΩ in series, 10uF to ground, another 2KΩ in series, and another 10uF to ground. You don't really need two poles of low pass filter due to your high frequency ratio, but I'm thinking there may be other things in that signal or other noise that would be good to stomp on. The signal to the micro would have a impedance of 4.05 KΩ and frequencies of up to a few Hz would be passed without bother. After that they start getting attenuated. 1 KHz will already be down by over 80 dB with stuff above that another 12 dB every octave.
Added:
As Kortuk points out in the comments, at these frequencies parasitic inductances and capacitances can matter. That is another reason I want two poles instead of one, even though a single pole would attenuate the 400 MHz plenty well enough in theory. I also wasn't planning on getting into this level of detail (I have a life and need to get things done. Every answer can only have a finite amount of detail. As a volunteer, I should have the right to decide how much detail I'm willing to get into) until Kortuk essentially called me out on it.
I agree that this filter should be implemented with SMD parts and carefully layed out to minimize stray capacitive coupling from input to output. It should also be physically close to the output pin producing the signal to filter. You don't have to worry about the output being at 50Ω and the first resistor being a mismatch, since the trace is intended to be only a few mm long.
The "down by over 80 dB" I quoted for 1 KHz is still valid. None of the stray stuff is going to matter at 1 KHz. The signal will drop from there another 12 dB per octave for quite a while, at least until well into the MHz range. Eventually the parasitic capacitance accross the resistor and the parasitic series inductance of the capacitor and the leads to it will make the filter work less well, and its gain will actually start to go back up with frequency. You'd have to look at specific part datasheets to get a better idea, but with decent parts and decent layout, I'd expect the bottom to be somewhere (factor of two easily possible) around 100 MHz. The gain at that bottom is so low that the rise in gain from there to 450 MHz should still be well tolerable, and the result good enough. These things are difficult to predict with any certainty, so to get real numbers you pretty much have to build it and see what you get.
However, I'd be real surprised if what I described isn't good enough for the job with significant margin.
Power rating on resistors is the primary difference but also maximum voltage rating can be lower on smaller packages. For capacitors, the maximum voltage rating will be lower on smaller packages and it is likely that the dielectric type will be "worse" for the same value in smaller packages. A "worse" dielectric means higher losses and/or capacitance drift with temperature.
For inductors, the effective series resistance will likely be higher on smaller packages but this can be traded-off by the inductor supplier using a higher permeability magnetic material - the downside being that it won't be very good in higher frequency applications. Also, smaller package inductors will, for the same inductance value as a bigger package, use thinner wires and thinner insulation on the wires and this can lead to lower voltage capability and lower self-resonant frequency due to the winding capacitance being higher.
Best Answer
0402 resistors might be rated for either 1/10 W or 1/16 W. You'll have to check the specs of the parts you're actually using (or, if you're buying off EBay or something, just assume 1/16 W).
Then you'll have to calculate for each one in the circuit, how much power it is consuming. For an analog filter, you want to consider the maximum input amplitude case.
If the power consumed is less than 0.5 or 0.75 of the rating, you are probably okay with 0402. If you power consumed is more than that, use a bigger part. You might even want to limit the consumed power to less than 0.5 of the rating to avoid self-heating causing your filter characteristics to drift, depending how repeatable you need the filter's performance to be.