Welcome trini,

Don't let me put you off but if this is your first CNC machien then starting with a large machine isn't always a good idea.
While it may seem like not much more work for large machine compared to smaller size this isn't the case in reality. There's hell of a lot more work and it's much harder to get a large accurate machine. Design and build quality become much more important.

Every thing Neil suggests is good advice you should listen too but please be aware of the scale of machine your undertaking. Just because others have built large a machine doesn't mean that machine performs good or couldn't be better with just a few small tweaks.!! . . . So Research then research some more then a little more for good measure before starting.