You can project as big or small as you like. For a bigger projection, move the projector away from the wall. Bare in mind that the further away you are, the fainter the image will be. Projecting onto shiny surfaces doesn't work so well, if you're not projecting too big then you can just stick a large piece of paper onto the wall and project onto that.
What you need (this isn't an exact science, you can swap bits out with whatever you have laying around:
20" length of 2x2
A magnifying glass (or camera lens, fresnel lens, magnifying sheet, anything that magnifies really)
A torch or bulb with minimal fittings
Sticky back velcro
The design might seem stupidly simple, and it is. But it didn't start there. It was originally more complex and I kept cutting back at it until it could literally be made by anyone with anything laying around the house.
The images on this intro show the finished project. It's projecting a hand drawn image of Mario.
Step 1: Test and measure
I tested with a macro camera lens, a canon 33mm DSLR lens and a magnifying glass, All worked perfectly fine. I also tested with a few different bulb types. I settled with a Philips spot light style bulb since it focuses the light in one direction.