The design guide for the ad:

The physical size of the ad in print will be:
90mm x 120mm ~3:4

The canvas size you'll need for 300dpi is:
1063 × 1417 Pixels

You can also use any 4:3 preset in your drawing program.
In GIMP I recommend Display - 4:3 - 2048 x 1536 (QXGA)

Remember to set the orientation to Portrait and avoid very bright, neon colors, as these cannot be printed correctly and will appear dark and desaturated in print.

#advert