1.That's a good size. Big enough to be useful, but not so big that you start to give significant construction issues. It's not really worth going much smaller unless you have to because of lack of space. The finished machine will end up around 300mm bigger than the cutting area in each dimension (give or take a bit).

2. By keeping the size down, you can get reasonable strength again without going to anything too ambitious by way of construction. You might end up with something that can cut aluminium with care.

3. As said, water cooling means that the spindle is quieter, and can run for hours without problem. So can air-cooling, but you need to be a bit more careful about dust management so that it doesn't clog. You are right about VFD. Generally these can be controlled manually via the front panel or connected to your control box for software control of start/stop/speed.

4. There are some spreadsheets on this site which will allow you to calculate the exact motor spec required. However, to be pragmatic about it, the answer for this general class of machine is NEMA23, 3Nm or 4Nm. Might as well go for the more powerful as there is barely any cost difference. Don't go for cheap eBay motors as these are generally high-inductance and for various reasons this is a bad thing!

5. BOB is break-out board, and functionally it is a kind of junction box that takes the connections from your PC parallel port and makes it easy to connect the stepper motor drivers and things like emergency stop buttons and limit switches (for end-of-travel detection). It does a little more than that (like provide isolation between PC and drivers for protection of one from t'other) and might also provide the signals you need to control the VFD but if you think of it as a junction box, you won't be far wrong. There are really cheap BOBs that will do the job. However, they assume that you are using a parallel port on your PC and a lot of modern PCs don't have these although you can add them via an add-on interface card. That brings up questions of Mach3/Windows, or LinuxCNC. That's another whole story but not really one that needs too much thought at this stage. There are other alternatives which complicate the story as well - we'll leave that one for now!

6. You will be astonished at how fast and how violently a decent machine can shake itself around. Even if you are not doing fancy 3D carving (and you will be able to) even things like boring holes involve frequent and rapid direction changes. That table will hold the machine while static but it's going to need quite a bit of bracing to handle dynamic loads.

Keep asking. Don't get upset if you get your leg pulled from time to time. Goes with the turf! But you'll get plenty of useful info as well.