How do I build HUGE constructions in Minecraft?

Rule number one when taking on a big project: Don't do it alone! Recruit some friends to help you out.

You can use a pixel based editor (Photoshop, Paint, etc.) to plan your constructions out. Each pixel can be one block and then you just design what you want.

I don't know of any off the top of my head but scripts can help if you plan to do vrey repetitive actions. Stuff like backing up while placing blocks to create a wall. I'm sure there are macros that would let you capture keyboard/mouse commands which you can use to build your structures.

Copy and Paste. You'll probably want to design the unique elements somewhere (in-game or in an image editing tool), then build them once in-game and copy and paste them in-game using a mod to create the large structures you're looking for.

WorldEdit has copy/paste support but loading it in Single Player requires some careful modifications. It's a bit easier to install if you're working on an SMP server.

For designing your project... Minecraft is really best. Be prepared to build a lot of temporary stairs to stand on as you test out designs. Or install a flying mod.