As i understand it 300 PPI is the typical for 'small' artworks especially for clean text resolution, however for small artworks the audience usually engages at a much closer distance. Therefore in the interests of keeping a bitmap file down to a reasonable size you can get away with a much reduced PPI for a large format/banner print. N general you should use vector graphics in the artwork wherever practical, and deliver final artwork to the printer in PDF or other vector large format digital printing. Your finished print will then be limited only by the output resolution of the print device. This is particularly important with text and line art visible pasteurization in the finished print will be very obvious and look amateurish.
Don’t go much above the minimum resolution you need, to keep file sizes manageable and avoid problems with the printer's RIP (Raster Image Processor the software that translates you artwork to the physical dots that will hit the paper). Beyond a certain fairly definite point, which I'll cover, increasing the file size does nothing to increase output quality of your online printing services.