If 300 dpi at 1/4 scale then it will make a final resolution of 75 dpi at full size which should be fine. I just tested myself to see using 202x78.5 inches at 300dpi. RGB mode, flattened, and saved as a tiff with LZW compression. Blank white file was only 16mb. Added a random fairly complex background so there was random image info and it made a 1.54gb file (3.99gb uncompressed). Converted to CMYK and saved again and that increased it to 3.86gb (5.32 uncompressed). So yeah, stick with RGB and I guess to an extent it will also depend on what the artwork is but with the LZW compression turned on and an RGB file you should have a file under 2gb.
Also as Colorcrest mentioned, RGB mode should be just fine for most any printer. In fact our entire workflow is based on RGB mode (we use the Adobe RGB 1998 profile) which gives us a wider gamut, smaller files, and generally more predictable results.