I'm 5'3, 115lbs and have been in Karv for over 3 years. It is hard to fight a larger person and nerarly everyone is larger than me. We spar at our school and tall guys with good arm reach have nailed me plenty of times. If you are hoping for one or two moves that will take a big guy out you may not find it but being small has it's advantages too. If you get inside try an elbow to the zyphoid. You can still cover up and throw a good knee too. I also discovered I am faster then the bigger people and they wear out faster than I do.
You will have to be fast and sneaky!!
Are there any other Krav schools in your area that do spar? You can't get good without it. I also agree you should round out your training with other training. I'm glad we have boxing and ground fighting!!