Smaller 30cm lamps will give you only about 30% - 25% of the energy you can get with larger lamps (read as: ~30-45 minutes or even longer cyanotype exposures...) and their useful lifespan are shorter by 50%. For 8x10" you'll need 10 of these lamps, spaced 10mm apart from each other, hang about 3-4" above printing frame's glass.

Regards,
Loris.