A 30" crate should be fine for the smaller end of the Cockapoo range (Toy and American mix) - anything 15" to the shoulders fully grown would be better suited to a 36" crate (Miniature to Show / Working)....... A fully grown dog should have sufficient room to stand-up; turn around and comfortably lay down sprawled out (as they do).
As for little pups - it's their DEN - the crate is the first place that they will pro-actively try / want to keep clean.
We would suggest buying a hook-on water bowl and 3 bits of "Vet-Bed" to the size of the base of the crate (one to be in-use - one in reserve and one in the wash).
Lots of people will suggest you partition off half of the crate for a early weeks - some will say don't leave a space for them to go to the toilet (to teach them to hold it !) - BUT our personal preference is to use the whole of the crate - so we suggest the "Vet-Bed" folded in half at the back with newspaper at the front in case they can't hold-on. They will quickly learn to keep the crate clean and go to the toilet when you put them out side in the morning - so when they have that mastered you can just use the Vet-Bed on it's own.
A puppy will be equally as happy in a 30" crate and they would be in a 36" - but if you get the smaller one then then you will ultimately have to buy a bigger one at some time and keep an eye on your dog for when it is needed - so if you go the 36" route from the outset - you are only buying one crate and don't have to worry about changing it (with the added bonus that puppy will associate that particular crate from the outset as it's special place).
We also have other suggestions - and they are only suggestions about crate-training so on - and if you would like one of our "Idiot's Guides" (we get lots of people asking us the same questions - so we put together a few sheets on the basics for people - from more of a physcological view point as opposed to the classic "do's and dont's") - then just ask and we can e-mail you one.
I was asked recently to buy a crate for a customer - and found a superb quality crate (36" with side and front doors) - a proper crate not a flimsy wire one - off E-Bay from a seller listed as "gwdavies28" - he appears to be a rep for Doghealth Stock nutrition in Dereham Norfolk.