It took me a while to get back to you, because this is a detailed question. My preference is for longer interconnects and shorter speaker cables. But, like everything else in life, there's a caveat. To have the longer interconnects, you must have equipment that can drive low impedance loads. As an example, if you are using a passive pre-amp, then you could not use long interconnects, because you'd need to be concerned with the capacitance of the cable. Or, another example: if you are using a computer running into a small DAC that uses a IC stage that does not have a current drive for long interconnects, you probably should not use long interconnects.
To help with calculating the treble cutoff frequency of your cable, this calculator may be a helpful start: http://www.sengpielaudio.com/calculator-cable.htm .
Concerning the speaker cable, is that the speaker itself possesses a very low impedance. Typically, you're talking from 1-16 Ohms impedance. Here, Ohm's Law prevails. The cable length itself also has an impedance. Both of these will begin working together. The cable will consume power from your amplifier, and one of the things you do not want is for your speaker cable to become hot, or in other words, to create resistance. As it creates resistance, you start having frequency dropouts of your speaker cable. That is, it will roll up or roll down.
So, how long should cables be? Is there a "rule of thumb"?
With true balanced, you can pretty much go any length from 1m to 100m. With RCAs, you need to be a little more cautious. I'd say anything from 1m to say, 20m should work fine. It also depends on the equipment being used and whether it can support that length.
With speaker cables, I'd say anything from 1m to 4m. Most of the high-end manufacturer's equipment I have seen should not cause an issue, so long as you pay attention to the gauge. Say, a 14 gauge down to 8 gauge, you should not have an issue. If you go the other say, say 16 or up, you need to start being concerned for the speaker cable length.