After test exports at 90 DPI (3952W x 2911H) on all pages of an illustrated book I've created to upload into Bookemon.com (was making sure I like the look of each finished page) I was right about to begin final exports into .png when I suddenly asked myself if I was doing the right thing given the book is for classroom use on a very large smartboard. Should I exporting at 90 DPI? Or double? Or triple, lol! Does it even make a difference? Would love anyone with the knowledge to please share it. Also, the same book will be viewed on laptops from the same Bookemon.com site which is even more perplexing for me to figure out.

Better save to pdf before printing. If you don't use much filter effects, that is the best way.
There are printers that can handle 1200 dpi with large formats, but you cannot export to such enermous resolution in big.

Thanks Lazur URH. I won't be printing any copies. Just uploading to Bookemon then using the i-book in the classroom on laptops and very large interactive whiteboard screens. My concern is loss of quality on the big screen. Is a 90 DPI .png strong enough for a screen 3+ metres long?

It all depends on the monitor's resolution, and wether you want to zoom it or not.
If it's just a fullHD, that you want to display an image only at 100% zoom level, 3952 for width is unnecessary.
4k UHD is 3840 pixels × 2160 pixels, so your current image should be cut off or scaled to fit to such screen.
Still, it's better to keep your drawing in vector format.
Who knows, what will be the next screen resolutions?

Presumable those large interactive whiteboard screens are high-res ones and are viewed from around 3 meters, it would all work nicely even with 50 dpi (or ppi / pixels per inch, while speaking of screens).

Also, it could be precisely calculated by this formula.

Thank you!