How to find SMD resistor and capacitor value and correct size
Smaller sizes tend to work better at higher frequencies and are easier to fit into a good layout.  Larger values tend to have high power ratings and (in the case of capacitors) have more stable capacitance under bias voltages. Larger are easier to solder (if you are doing this by hand) but take up more room.
If you are a beginner and soldering this by hand, 1206 or 0805 are good sizes to use.  Smaller is not too much more difficult to work with, but no sense making things harder for yourself unless there is a good reason.  If you are paying for assembly, and they can do smaller sizes, this may not matter.
When picking resistors, pay close attention to power ratings.  Don't exceed the maximum dissipation.  If you do, pick a bigger size.
When picking capacitors, pay close attention to the change in capacitance with voltage and the impedance at high frequency.  For example, here is a random 1 uF 0402 capacitor:

And here is the same 1 uF but in 1206:

(images from:  https://ds.murata.co.jp/simsurfing/mlcc.html?lcid=en-us)
Comparing capacitance at 5v, the larger package works much better, but it also has higher impedance above ~ 1 MHz. At higher frequencies, a larger package size may not be appropriate.