I've decided to build up a fixie and I'm trying to figure out gearing. My main question is how hard is it trying to control your speed down or how easy it is to kinda just let your feet kind of float in place on the pedals while spinning. My big problem is that I have some rather large hills that I'd rather have low gearing to climb, but I'm not sure if I should go with the highest gearing I can to be safer on the down hills.

P.S. When commuting on my mountain bike I can regularly get up to speeds of ~30 mph / 48 kmh