Desi is rapidly growing out of her 36'' cage she can lie down in it but can't stretch right out anymore and when she sits up she almost touches the top.  She is now nearly 10 months and I think it is necessary to get her a new cage but I am unsure as to whether to get the next size up the 42'' or go for the 48''.  I would really appreciate any advice on this.
thanks

I personally would always go for the biggest you have room for - even if that was a bit big for her at the moment. So my choice would be the 48" although as i said just my opinion :)

Definately go for the largest you can find. I have a 48" one in my living room which the 2 pups sleep in at night. They are 8mths and 6mths and have plenty of room to stretch, stand up and unfortunately, play in. :D And Tia (13mths) also climbs in beside them when the mood takes her, or will go in on her own for peace and quiet during the day. She doesn't sleep in a crate, she just likes them :D The 42" is just a bit small for my liking, although it is only 6" difference in length, there is a great deal of difference in the overall interior space
Kay

If you are going to want it to use in the car and staying away then the oen she has now should be fine.  She is unlikely to get any taller only fill out so it wil be fine for this.  any bigger and it may be awkward to fit, and too heavy to carry.  Two Elkhounds fit in and travel in a 36 inch cage, and were the biggest size they did on the coaches so some pretty large dogs had to make do with them, including Jap. Akita Inu and Chessepeak Bay retrievers

If you want one as her den at home and prefer it to be roomier then get the size that fits the sapce you ahve, as you won't be folding and carrying it so much.

If she's trained and you're just getting it for when you're not home, or her own private space, go with what you can find room for.  Meatball went through 2 cages as he was growing and the third I got fits them all.  :) 

I used the cage for toilet training but she also sleeps in it at night at which stage I bolt the door.  I would like to start leaving the cage door open at night for her and during the day so she has access to the rest of the home but am a little nervous to do this.
I am thinking perhaps that the 42'' is the way to go as we do not have that much space in the second bedroom and I am worried that the 48 might be too big. However I want her to have plenty of space.

My 3 elkhounds travel quite comfortably in a 42'' crate...so I would think it would be fine for 1 lab
Archer

I got a 48" for my Weimarner and found it too big so went for the 42" and sold the 48".

Most of the dobe owners I know use 42 inch crates so a 48 inch crate for a lab would be a bit of overkill.  Like archer I use one that size for 3 Elkhounds at shows.  I find I can get two of mine in a 36 inch crate, and they would fit in the 31 inch if it was taller.  For one dog I bought one of those round cornered crates in the 31inch size, as they are taller (26inches) so just fit one Elkie.  The one I use in the kitchen is a cross between the 42 and 36 inch crate as it has the length but is the width and height of the 36 inch one, so you may find that you can get a size between the 42 and 48 as you don't need all that height (which is linked to width but not length).

My lab is in a 35" crate and she does fine, infact at a pinch I can get 2 of them in there, it even has a divider if I wanted to keep them seperate. Truffle is still shut in at night and she is now 2, almost trust worthy but not quite :D Alos I know that as soon as I stop shutting her in I will have 2 Labs sneeking on to the bed at night.