You will really mess up your wine. You need as little O2 as possible. If nothing else fill it with CO2. or find a smaller container. Get yourself a 3 gallon carboy or Better Bottle.
Do not worry.
As long as fermentation is still happening CO2 is produced
and your wine willhave a layer of CO2 on top of it.
Now when fermentation ceases then you have to be carefull.
The carboy will be filled with CO2 still from the fermentation
but when the airlock might get negative (due to higher air pressure) air might get into the carboy.
This might accumulate and the CO2 in the carboy might get less and less dense and air might take over.
So when fermentation ceases best is to rack the carboy
(you should do that anyway) in a smaller carboy with no air
above the wine.
I agreee with the previous posts. You want as little air space hitting the wine once fermentation is complete. A day or so may not harm the wine but I would transfer it to a 3 gal carboy.
I have a bunch of 1 gallon jugs that I use when I have less than a full carboy.
IMO, you're better to divide the batch into 3- one gallon jugs that are fully topped up, than to leave a large surface area exposed in a half full carboy.
I've got a bunch of the 4 liter Carlo Rossi glass jugs from a local bottle redemption place that make great secondaries for small batches Use a #6 bung and you will be very happy:,