Is there some command or preferences dialog somewhere that will tell me the size of the virtual address space that Windows has set aside for usermode applications?
As far as I know there is not. But you can determine it logically. Every user-mode application has 2^32-cell address space, which is 1byte per mem cell and 4Giga rows = 4Gigabytes, for 32-bit O.S. In 64-bit O.S. theoretically that space is 2^64-cell but todays usage is not that large, 40 bits if I'm not mistaken. That space is for application itself, for program code and stack area. For heap, it is basicly limited to system's virtual memory space which of course can be as large as HDD's capacity.
You can check the VM space in Computer Properties -> Advanced tab. And also you can observe it in Task Manager.
As an example one applicaiton of mine was calculating Morris Series. I did configure VM size to 1TB, locating pagefile.sys in a 1TB disk. Application started from 1K memory usage crashed after a string variable become a 1TB long and that was the furthest I could go with that particular coding.