Hello and welcome to BYC! 
You definitely want to build it bigger than you think you need. Chickens thrive in the biggest space you can give them without crowding. Make sure it's big enough you can walk into, this is helpful for you and gives the birds plenty of ventilation. On the ventilation note, birds should roost low, ventilation in your eaves, 1/2 to 1 Sq foot vent space per bird.
Best wishes with your build, ask as many questions as needed!

You definitely want to build it bigger than you think you need. Chickens thrive in the biggest space you can give them without crowding. Make sure it's big enough you can walk into, this is helpful for you and gives the birds plenty of ventilation. On the ventilation note, birds should roost low, ventilation in your eaves, 1/2 to 1 Sq foot vent space per bird.
Best wishes with your build, ask as many questions as needed!