1. Print to PDF in even smaller size (0.625m x 2m) and let the print shop scale it up 4 times on the printer to get the required print size (2.5x8).
2. Split the artboard and design in 2, each 1.25x2 metres, and let the print shop scale it 2 times and attach the two parts together.
3. Create 4 artboards, each 2.5x2 metres and split the design into 4 parts, which combine together into the final 2.5x8 banner while avoiding any scaling.