Bleed and ppi for large mesh print

Where is the print being installed? Indoors or outdoors? How will people typically be viewing the print? Will they be able to walk up and view it at a close distance or will they be seeing it while driving by in a vehicle?

Either way, 300ppi at full size is sheer overkill. Such a thing would indeed result in stupendously large file sizes. Raster-based imagery at 150ppi is good for a poster. I'll usually set raster imagery and effects at 72ppi on vehicle wraps. Billboards are often set at 25ppi at full size (or 300ppi at 1" = 1' scale). Resolution and raster effects settings need to be tailored for the print's viewing environment. Due to the size of the mesh graphic you might have to design in scale rather than full size (unless the print company can accept Illustrator files in large canvas mode).

The kind of bleed needed depends on how the mesh print will be installed. Will it be installed in some kind of frame or cabinet? Or will it be used like a large banner (edges folded over, grommets added)? The print company might be able to help with those details.

And be careful with setting the downsampling ppi when exporting to pdf.

Although you probably don't need 300 ppi (that is for the reading distance of a high quality magazine) you want to control the downsampling. There is currently a bug for large canvas documents that causes images that are downsampled to have3 a ppi 10 times smaller than requested. So enter a ppi 10 times higher than needed.

See:

https://illustrator.uservoice.com/forums/601447-illustrator-desktop-bugs/suggestions/47439314-pdf-files-made-on-large-artboards-get-downscaled-i

These are questions for the printer.

What elements are included that have a PPI? Raster images, or effects? 300 PPI (usually) would be absurdly high for a 20-metre print.