I'm looking to upscale the attached image so I can print it on a 90cm by 60cm canvas (36in x 24 in roughly). If I want the image to have 240 pixels per inch (is this too much?) Id have to get the image to 5669 by 8504 pixels in size. If I do this I get the error message that the file created would be too large (500Mb approximately). Can anyone help me get this picture to an acceptable resolution at the size I want to print it?
You can change the maximum file size in GIMP's preferences
Click Edit > Preferences, and under System Resources increase the "Maximum New Image Size". Note: this may make GIMP run slower, depending on your computer system, and how big the file is.
As for upsampling (resampling), it will degrade the quality, and I'm not sure if I would recommend that. It would really depend on how much you are resampling the image by, and what the original image size was. Small low resolution images will not scale up well. They will look blurry. If the image is already quite high resolution (large in pixel dimensions), then you might get away with it. Perhaps try adding some sharpening too.
If the original image is the size of the sample you posted above, then it's already a 10.7 megapixel image. That's probably already good enough for a large size print, as long as it is being viewed at normal viewing distances, and not close up.
If you don't want to risk that, might I suggest 150ppi is probably good enough to rescale the image for a large print. Then add some unsharp masking. This is at 100% view (click on it to see it full size), you can see the rescaling I applied in the dialog.
See this related question here: What resolution should a large format print for artwork be?