For me it really helped to configure Gimp for pixel art: set the pen to 1 pixel, save some palettes, set the grid to 8x8 when doing Spectrum graphics, a keyboard shortcut to show/hide the grid, etc. Gimp defaults are not good for pixel art.
Other than that, jut zoom as needed. I usually scale the games x3 when trying a retro style, but in Gimp I draw mostly at x8.