Fires jump roads, main highways, sometimes. That would be at least 200 feet. Say if firebreaks were every mile and wide enough to stop most fires, and the forests were cleaned out of deadwood, low branches, and other ground fuels, just for consideration. That would cut forested area by 10%, but would probably have other benefits besides making fire control a lot easier, such as improved wildlife habitat. Maybe an environmentalist could do something actually useful and design a national firebreak system on his computer . . . optimum spacing, optimum width, that kind of thing.
Then, of course there are tundra fires, which Alaska gets every summer, huge fires, lots of smoke that travels hundreds of miles but burns only a few illegal cabins and no loss of human life usually.
If we do suffer a terrorist arson attack super firestorm, you can bet that a national firebreak system will become a priority very fast.
The 10% given up to the spacing can be leased for farmland, parks, etc.
On 2nd thought it won't fly. Such a system runs counter to the enviro dream of totally connected wilderness zones, with field mice and grizzlies roaming in continuous forest land corridors from Canada to Mexico. That is Holy Writ to them.
Consider Camp Pendleton Marine Base between San Diego and LA. In time of war, with Marines facing combat, the enviros permit the Marines to practice amphibious operations on 500 feet out of 17 miles of the base's coastline, lest the brutal ugly jarheads disturb the Lesser Western Green Throated Blue Balled Sand Flea.