Is there any drawbacks to learning guitar on two different sizes at same time?

I am just learning to play guitar (taking 6th cl this week). I have a full size adult guitar, but have thought about buying a 3/4 size to put in the trunk of my car. This would allow me to practice on my lunch hour or wherever I happen to be. I have a small car so trying to practice using a full size gets a little tight. Would it hurt my learning to practice on the 3/4 sometimes and full size at other times?