The usual way for printing is adjust the printing resolution using Image -> Print Size US letter is 8.5" x 11" but your printer (usually) requires borders, so change units to inches, and in this case Height rules so set the height to maybe 10.5, whatever you think suitable. Starting with a small image the print resolution is now equally small and the print is going to be poor quality. All depends what you are printing, packing case label or the Mona Lisa. Screenshot: https://i.imgur.com/Xyg1eHr.jpg
The other way is open a blank USletter canvas, there is a preset File -> New -> Template Open the small jpeg as a layer File -> Open as Layers
Use the scale tool and drag the jpeg layer to size. Use the corner 'handles' to keep aspect ratio. Screenshot: https://i.imgur.com/J5a0qLh.jpg
These shown in kubuntu 18.04 / Gimp 2.10.22 but Gimp 2.8 is similar. Can you install the Gimp-Gutenprint plugin? (all depends if the printer is supported) That can scale a image for printing.
Edit: Pulled the 2.8 (portable) out and are you saying that this does not work with regular linux CUPS print ? Set the paper size / Set one of the (W / H ) image size
The other way is open a blank USletter canvas, there is a preset File -> New -> Template Open the small jpeg as a layer File -> Open as Layers
Use the scale tool and drag the jpeg layer to size. Use the corner 'handles' to keep aspect ratio. Screenshot: https://i.imgur.com/J5a0qLh.jpg
These shown in kubuntu 18.04 / Gimp 2.10.22 but Gimp 2.8 is similar. Can you install the Gimp-Gutenprint plugin? (all depends if the printer is supported) That can scale a image for printing.
Edit: Pulled the 2.8 (portable) out and are you saying that this does not work with regular linux CUPS print ? Set the paper size / Set one of the (W / H ) image size