If you're wanting to make your legs bigger then yes a rowing machine is a good exercise machine and exercise for making your legs bigger.
Rowing machines are also good for weight loss, muscle toning, upper arm toning, glute toning etc.
Rowing machine exercises are also good for losing belly fat and giving yourself a good cardio workout.
I use a rowing machine 30 minutes per day everyday and I've lost belly fat and lost a lot of weight and feel much more energized as well.
My legs have gotten bigger and my muscles have gotten stronger as a result of using the rowing machine.