Particular will give this message to prevent you from causing crashes with too large particles. It should not stop you from working with large images, though. If it's an option, scale down your particles by creating a pre-comp with smaller dimensions. This will get rid of the message and shoiuld also render faster. If for some reason you can't follow that suggestion, try using AE's standard 3D layers and arrange them using the 3D Assistants (the lite version of which comes free with AE).