Is it possible to run Snow Leopard (Server) inside VirtualBox on a Mac running Lion? Is it possible to run Snow Leopard (Server) inside VirtualBox on a Mac running Lion, as a means of continuing to run legacy PowerPC apps (Eudora, Quicken, et.al.) under Rosetta, even on new Macs (Lion only)?

The EULA seems to allow running the OS in a VM on Apple Mac hardware.

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If so, what VirtualBox configuration and/or settings should one try?
How big a virtual HD should one use?

(My goal is to run authentic vintage apps, if possible, not work-alikes).
Yup. You should be able to. Give it a try. :) 

Though, I hope apps will be updated to work on Intel, because this isn't the most resource-firendly solution. It probably takes more processing power / RAM / Hard Disk Space to install and run Snow Leopard than to run the app itself. 

Yet, I think you can find alternative Intel apps. I really like using Apple's Mail for e-mail, and this article shows several alternatives to Quicken. 

Anyhow, its your choice. Give it a try and see what works better for you.