I've started work on a large canvas 91x122cm (36"x48"). The large whiteness scared me at first. That's normal. I just grabbed the canvas and start sticking old book pages on and felt much better. My mind is working faster than my hands on this one. Probably due to the size. Its a bit frustrating when you want to get to the next stage, the next layer, or try the next idea and you still have such a big area to fill before you can go on.
This collage will be based on the old wooden chairs I bought second hand recently. The canvas is big enough to make the chairs nearly life size. Its going to be all about the background with the chairs to add interest to the foreground. These ideas could change as I go through the creating process but that's the joy of it, letting a piece evolve and not forcing it to go where it doesn't want to.
Its funny because these are the things that are frowned upon when you study art formally. Its meant to be planned out completely, doing little tests and mock ups until you get it right. No intuition or going with the flow is ever encouraged. Maybe that's what you have to go through to learn the principles of composition, design, colour and once learned you can go on what feels right and follow what you heart tells you.