Upgrading crate to larger size: when to make transfer, how to cover metal

Question
We have a 10 week old Golden who is well adjusted to the plastic crate that we used for our Cocker Spaniel.  However, I know that we will need to get a larger one before long.  This crate is 15" wide, 24" long, and 16" high. At what age is he likely to outgrow this crate and what size should we purchase?  Also, we need a crate that can fold up so we are looking at a metal crate.  What do you suggest to cover the crate that he is less likely to pull in and chew on?

Answer
I start my puppies in a 24'' plastic crate and move them to a 36'' one when they start looking cramped in the 24'' one.  I may have moved Holly out a little early the last week of April when we kept a 7 Week old Golden for friends.  

I like the plastic crates for travel.  I nest the halves and put out suitcase and everything else in it.  That works very well when we share rides with friends.  You can even buy ones with wheels eliminating the need for a baggage cart.  I am not sure why my friends brought the little Golden in a huge crate, a folding plastic one by Nylabone.  It was much easier to fold than any metal one I ever saw.  I see a bunch of crates, because we frequently keep dogs for friends and usually I like their regular crate.  

If you cover a metal carte, you need to choose something too stiff for the puppy to pull it in.  Perhaps a carpet remnant or throw rug.  Just be sure to cut some holes for ventilation and for the dog to look out.