You could store it all as a single image but it could only be so large, and you'd have to break it down into separate textures because there are limits to texture sizes. You'd need enough memory to load the whole thing in at once. With a 32 mb graphics card you could do for example a 2560x2560 32-bit superbitmap thing. --- about 26 megabytes.
If you're running at 640x480 that's about 4 x 5.3 screens of play area (about 21 screens worth). Typical platform game levels could with tilemaps are usually quite a bit larger than that. With 64 megs of graphics mem you could get up to around 40 screens. If you're talking about layers you're going to have to think about either much smaller game worlds or using tiling for at least part of it.
If you're running at 640x480 that's about 4 x 5.3 screens of play area (about 21 screens worth). Typical platform game levels could with tilemaps are usually quite a bit larger than that. With 64 megs of graphics mem you could get up to around 40 screens. If you're talking about layers you're going to have to think about either much smaller game worlds or using tiling for at least part of it.