Wizard vs. Tabs
Use a Wizard, when you want to guide the user through multiple requried steps.
Use Tabs when the user can select an arbitrary tab, make changes, and then commit them without looking at the other tabs.
I always find “Tabs with Back/Next” awkward. I understand the idea to provide a little “ramp” between novice and advanced users, but in most cases I’ve seen, the design of the individual pages isn’t suitable for novice users.
For a wizard, it’s not just size – the user should have the impression he interacts with the same window all the time. This is commonly achieved through a header and other control elements in the same position throughout all pages, and same size.
Example: Installshield violates the “same window” metaphor pretty badly (though it’s not too obvious): each page is a new modal dialog, centered on the primary monitor. So move the window to another position, click next, and bam, your window is back where it was. Aaaargh!
In a wizard, a changing form size is quite awkward for users. It’s like holding his hand, but you are constantly dancing around him. In more serious terms: the user action (clicking “Next”) has an unexpected side effect (“Window size changes”). This decreases the users perceived control over the software.
I’d argue that even for tabs, changing sizes are awkward. First, it looks shitty – which subjective. Second, if you position the window containing the tabs so that they are in the corner of a screen to reveal another document on your desktop, an you switch to another page, either part of your dialog vanishes, or it jumps further into the desktop, potentially covering other things.
(Full disclosure: I am working on an app which does that, and it’s my fault. Watching users pull around that poor little dialog makes me cringe. It’s one of those little speed-bumps that build up annoyance).
If the choice is between “a little crowded” and “almost empty”, it’s usually fine. Empty space looks good. Just having a large prominent window with a single input field makes a clear statement: this is your next step, and it’s darn easy. That’s exactly the message you want to send with a wizard.