Seems the length you need is dictaed by the size of what you might strap. A bundle of firewood is going to require longer straps than a bottle of wine.

You can employ some basic geometry (e.g,. 2 xPi x r=circumferance of a circle) to get a rough idea of what a strap length can accomodate.

Personally, I use bungees for my tent on the rear and store-bought compression staps with locks on them for my sleeping bag on the front rack. Don't know how long the latter are. The bungees I have used for firewood and for runs to the grocery store where I have put some items in my sleeping bag stuff sack.