Anyway VMWare now has a promotion for their VMWare Fusion for Mac. Unity works much better than Coherence, and VMWare is a big name company with decades of experience behind them, so you might want to check them out.
Unfortunately from my experience Unity doesn't work anywhere near as well as Coherence. Sure it's nice that Unity separates the windows so that they can be minimized and used with Exposé, but you can't really make use of programs that have taskbar icons and overlapping windows don't even display correctly in Exposé. I really wanted to give VMWare a chance, so I waited and waited for them to add the features of VMWare Workstation along with some other cool features, but it never happened. Fusion still doesn't have multiple snapshot support, and Parallels now does. Considering that VMWare Workstation has had that and many other features that Fusion still doesn't have for a while now, I see it as a sign that VMWare is not very much committed to the Mac platform.
Think about it; if VMWare was really committed to developing for Mac OS X, they would have been able to come up with a product much more well featured than Parallels by now considering the resources available to them.
That said, if I didn't already have Parallels and was considering whether to get Parallels or VMWare right now, it would be a tough choice. $40 for VMWare Fusion is a really good deal and being half the price of Parallels Desktop might just be enough to change my mind and make me go with Fusion. On the other hand, Parallels has said that they will add all of the features that VMWare Fusion currently has over Parallels (64-bit OS support, multiple processor support, Exposé support for Coherence) in updates to Parallels 3.0. So go with Parallels if you want more features, VMWare if you want better cost and support.
Getting back on topic,
1) 30 GB (the default) for Parallels, 15 GB for Boot Camp
The size of the partition for Parallels doesn't really matter, as long as you make sure that it is big enough to store everything you want to put on it.
2) I do, but you only need to do so if you want to be able to run Vista, play 3D games or have complete compatibility, otherwise the Parallels disk will be enough. If you want you can also do the reverse; have a Boot Camp partition that you can boot in Parallels but not a Parallels virtual disk, the disadvantage is that you can't suspend or make snapshots with a Boot Camp partition in Parallels.
3) I'm not sure if this is possible with Parallels 3.0 virtual disks, which is why it is a good idea to make your virtual disk as big as you think you will ever need it to be, because virtual disks only take up as much space as the data they contain.
4) You can run Parallels Compressor to compact your virtual disk, for example if you delete data and want to recover that disk space from the virtual disk.
