Binary package hint: debian-installer
The default swap partition size should be equal to the amount of RAM. This is the behavior for the live CD and for the alternate CD.
I installed kubuntu-alternate-amd64.iso (from 2011-04-13) in expert mode and followed this test case:
http://testcases.qa.ubuntu.com/Install/AlternateExpert
I chose the option for a separate /home partition and chose to encrypt the user's home directory. The size of this partition is also automatically determined. I did not chose any of the sizes manually. The size of the disk is 8.6GB (Virtual Box default). This seemed like it should be enough to accommodate 1GB of swap (amount of RAM) plus the root and home partitions without having to shrink the swap partition. If the partition did not fit, a warning should indicate that > 8.6 GB is necessary to install.
This is because 8.6G is too small to allocate 1GB of swap space when /home is installed on a separate partition.
I tried with a 30GB disk drive and 1GB of RAM and 1GB of swap space is allocated.
However, I confirm that with the default drive size in virtualbox (8.6GB) the swap space is only 333MB and there's no warning that less swap space than RAM available is allocated.