If it's a raster image you made in Photoshop, it's impossible to increase the size of a raster without sacrificing quality. Though if the image is to be viewed always from a distance (like a billboard), then you can lower the resolution to as little as 9-12 dpi and it would look fine from at least a few yards away.
If it's all vector objects, they are infinitely scalable to any size. There is no resolution quality to worry about with vectors. If you need to use a raster image that isn't very complex, you could trace the image in Illustrator (look up Live Trace and Live Paint) to make a vector version that you can resize.