for digital printing, I like the final file to be somewhere between 100 dpi & 300 dpi.
On average size stuff (2' x 3') I might go with 150 dpi... unless it is a photo & is designed for up-close viewing... then I might go with 300dpi.
For smaller 1' x 2' truck doors, I'd go with 300 dpi.
For any large banner or sign (3' x 4' and up) I will usually settle for 100dpi, as the higher res files will only cost me time & hard drive space... but for distance viewing, it won't produce any real difference.
As for a specific answer to your question, I would always design at actual size & design at least as high of a resolution as you want to send to your printer.
In some cases if I am designing a logo with 3D effects & several layers & lots of my time... I may have a job to make a small sign today... but i may need a larger sign in the future, so I design larger then I need (and/or at a higher resolution) ...then for today's job, I will save a smaller version to speed up ripping their small print, & keep the larger file on hand if they want a big sign later.