Rule of thumb is big bikes are better on road. small ones are better off it. Every trip will be a mixture of both and you have personal preference to take into account as well. To my mind the sweet spot is 500-800cc for a long, mainly road, tour - but for a two-up journey you might want to be at the upper end there. I really don't think anyone needs over 1000cc but as I say, personal preferences. Reliability shouldn't be an issue on a trip of a few months unless you're starting on an old nail (engines are usually the last thing to go wrong).
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