Our general rule of thumb is that artwork containing bitmap images should be supplied at a minimum of 300dpi at quarter scale.
For really large artwork, where files are getting too large to work with, you may get away with a smaller scale or resolution.
Remember that large format print is generally viewed at a distance, so it doesn’t need to be as high quality as a printed brochure, for example. However, please avoid sending us artwork used on web-sites, as they are generally far too small for large format print.
Vector artwork is ideal where possible, as it can be scaled to any size and retain its sharpness.
When scaling artwork, please keep it to a round scale (eg 25% or 50%). Let us know the ratio and output size, too, so that we can scale it back up correctly when printing.
Also, be sure that your dimensions are in proportion. For example, A4 artwork can’t be scaled up to a 3 metre by 1 metre banner without us having to alter the layout of your artwork.