if I say 'one leg' do I say una or uno as pierna ia 'la'?
gender of word question
No, you should use la since the parts of the body are not personalised in Spanish in the same way as they are in English
In English we say my leg ......but in Spanish la pierna as in the context you might be telling the doctor that your leg hurts In Spanish it is assumed that you are referring to your leg unless you indicate otherwise
Doctor, I have a pain in my right leg
= Doctor, tengo dolor en **la pierna derecha**
or Me duelo en la pierna derecha = My right leg hurts
I would encourage you to look at Paralee's excellent lesson in Lesson 4:9 You may not be ready for the general teaching yet but you ill be able to view the Pictures which show the correct way to say that a part of your body hurts.
edit*: My mistake, Feliz is correct.