.. got the economy, power to fight uphills and mostly wind, size/comfort, and confidence for its endurance.
but, upon reflection, it is a very complex question .. as shown by the answers so far.
time, distance, company, expectations, destinations, mindset, costs, economy, power, comfort, etc, to name but two
.. also there is the age-old question of `offroad` or `not`.. which can completely change the equations.
.. also this would change choice of tyres, and their expectations of use and wear properties.
I noticed that some riders off here, ABR, rode some big bikes down to Spain/Pyrenees/France or wotever, recently .. did some fab trails and then rode back to UK .. brill!! .. that's the way to do it.
But off-road is not for everyone. So if its tarmac only, it makes sense to do it on a larger bike, that will eat the miles, give comfort and power and save time. I know I did it on a 1200, 1000, and 750. .. but my first time was on a 250 .. and that has always been the most memorable! .. perhaps because it was the first time .. but also because the bike was more usable for evening towns and local day trips.
Nowadays though, the big manufacturers are producing bikes, like my NC750, that have power and comfort .. and economy, as well as ease of use and practicality. So the old assumptions are a bit out the window.
Also, there is the 125 phenomenon, that gives excellent economy but still a decent turn of sustained speed well into the 50's?
Then, addressing the question of `how small you would go` .. well, as already answered, it depends on time and mind set. I have had great fun on a 50cc bike .. the comfort was fantastic, and the economy the same.
Its just you live in a different world to say a 1200cc rider.
But that choice of world is yours.
