Use a measuring tape to figure out the circumfrence.
If you're just laying a ruler across the width of your arm or leg, it tells us almost nothing. We have no idea where you're measuring.
Lower leg? Your calves? Be specific. Actually, there doesn't really need to be anything to this. If you look out of proportion, then make something bigger.
If your skinny you shouldn't really be worrried about specific body parts and targetting them cos it wont work well. If you want to get big you should focus on basic compound exercises e.g squats,deadlifts and bench press.
But in answer to your question everyones body is different and its really hard to say how big what should be.