I was also wondering, for reds or whites, if people have found the
> results to be better by using a 19 l carboy instead of a 23 l carboy,
> producing a more full bodied wine, but also with a higher alcohol
> content. Are there any wines that are more or less likely to benefit > from this?
From what I've read, kits are balanced so that when reconstituted to 23L the acid/tanin/pH are in line to make a balanced wine.
There are instructions in the archives about how to improve kit wines, they are probably worth a read at least.