GIMP Version: 2.8.14
Operating System: Windows
OS Version: 10
GIMP Experience: New User

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is a 90 foot long canvas possible?

The maximum canvas size is too small for what I need... is there a plugin or hack to increase the canvas size?

Yes if you have the hardware for it yes

Create a new image File > New..., change pixels to "ft" but know that you will have an alert from GIMP, telling you than the image will be 1.1 terabyte and that just the first layer...
but if you have the hardware that can support such an image, like terabyte of RAM with 1000 cores, etc... > yes, you can create that 324 000 pixels wilde image
(maybe upgrade your GIMP to 2.10 before as you are on a very old 2.8 version)


BTW I don't want to discourage you, because you are on Microsoft Windows, but the top 500 super computers on this planet are on Linux for near a decade now https://www.top500.org/statistics/details/osfam/1/

90 ft but what is it for ?

If it is for a banner, then you should consult a specialist in printing banners where you might be told.

Supersized graphics 50 square metres+
30-50 PPI (DPI)

So in Gimp a 90 ft x 5 ft banner @ 30 ppi might be 32400 x 1800 pixels Large image but not impossible.



edit: Although thinking about it, a vector program such as Inkscape might be better.

and you might want to invest in one these.

https://www.techspot.com/news/103607-mo ... isc-v.html