As Scooter suggests, if the wine is actively fermenting then the yeast will be producing enough CO2 to blanket the top of the fermenter so you don't need to top it up at this time. When active fermentation ceases and when you are ready to rack then you might choose a smaller container or you might top up the secondary. Assuming your recipe was designed to provide a volume and SG with an expected ABV then you may not want to dilute the wine with water. Adding more of the same juice or using a similar wine to top this up (or grape juice) are options but so is using say, glass marbles to increase the height of the wine... The secret, in my opinion, is to ferment in a bucket (your primary) and start with a larger volume than the volume you intend to bottle. That means you are never concerned about how to solve the problem of excess head room. Your "problem" is what to do with the extra wine that will not fit into your secondary... and here I use mason jars that I store in the fridge...