A sign printed that large will certainly not be at 300dpi (the machine would never spool the print).
I know my sign guys work at artwork supplied at 1/5th or 1/10th printed size. If you can export a vector as PDF the size of the artwork will make no difference as long as it is proportionately correct.
The bigger issue you will have is that most artwork will be required as CYMK not RGB. Conversion to CYMK will need to be done post export.