HELLO, I AM BRINGING MY SMOOTHCOAT CHIHUAHUA HOME A WEEK FRIDAY.I
HAVE EVERYTHING EXCEPT  A CRATE FOR WHEN VISITORS COME TO HOUSE.IM AT A LOSS OF WHICH SIZE, ALTHOUGH HE IS LITTLE, IS IT BETTER TO BUY A MEDIUM. OR BIGGER SO HE CAN HAVE LOTS OF ROOM WHEN WE GO INTO THE GARDEN.AS I HAVE TWO OTHER DOGS.AND I WANT TO INTRODUCE THEM GRADUALLY. HE WONT BE IN IT AT NIGHT AS I HAVE ANOTHER CARRIER FOR THIS.  THANKYOU FOR YOUR ADVICE.

I'd say as large as you can afford/have room for.  If you want to use it for toilet training purposes too, you can always reduce the area availalble using cardboard boxes (although I'm now of the opinion that a dog will only become dry/clean when it is physically ready, and the use of a crate before that time is possibly not helpful).

However, I'm a little concerned that you feel the need to put the pup in a crate when visitors come to your house - you should be using these visitors to help socialise your pup, not shutting him away from them - he may even start to act up when you have visitors if he's always crated :(