Summergirl,
I'm relatively new to this also (only done 3 kits), but I've spent a lot of time reading this forum. I'm sure others will chime in, but I'mpretty sureyou are ok to leave some headspace in the carboy at this stage. Since fermentation is still underway and you have not degassed yet, your wine will be protected by a blanket of CO2.
You will need to top-up the carboy after you degass and stabalize, as there will be no more CO2 to protect the wine from exposure to oxygen.
You may also want to consider topping up with a cheap bottle or two of similar commercial wine. Some feel topping up with water can dilute the wine.