Resistor size

[arpruss]

When I used a 1M resistor for attenuation, I could barely register a 3.3V input. I got about 50% attenuation with a 100K resistor (though also some signal distortion). Are you sure you meant 1M?

[pingumacpenguin]

It depends on your source. 1M is a "safe" value. If your source is high impedance then 1M may be too high. If you are sure that your source is not going above 3v3 then you can skip the resistor all together.

The resistor is a lazy way of ensuring that there is little chance of frying the input. In a real scope, there would be a clamping circuit, buffering and perhaps some MOVs or whatever to keep the ADC safe from being fried by the input. No attempt is made to match impedance with the source, I leave that as an exercise for the user.