This is actually one of the more common questions I think
It really depends on what you plan on doing as far as future tattoos go. If you're someone who doesn't see themselves being a 'tattooed person', meaning it's one of your defining physical character attributes, but it's your only tattoo or one of very few, the proportions are directly related to the significance in meaning the tattoo has to you.
If you are a 'tattooed person', it's really just about real-estate and plans at that point.
If you're someone who has existing tattoos and an artist you use more than anyone else, I think it's one of those conversations to have with that artist, as he knows better than anyone else what you're trying to accomplish with your canvas.