More than 400 firefighters continued Wednesday to battle a massive wildfire burning out of control for three weeks in the Richardson Backcountry about 60 kilometers north of Fort McMurray.

"The length of this fire is about the same size as it is from Edmonton to Red Deer," said Sustainable Resource Development spokesman Geoffrey Driscoll. "It's got about a thousand kilometres of perimeter on it, so it's a big fire right now."

The fire is currently 416,000 hectares in size, making it the largest to start in Alberta since 1919, Driscoll said. The fire started on the same day as the Slave Lake fire, which destroyed 30 per cent of the town on May 15.

Dry conditions and a lack of rain have caused the Richardson fire to grow to its present size, Driscoll said.

The majority of firefighting resources are concentrated on the fire's southern edge to keep it from spreading to nearby oilsands sites and Fort McMurray.

Heavy smoke doesn't mean the fire is getting closer to the northern Alberta city, but people should be prepared in case they need to leave, said Rebecca Wade with the Alberta Emergency Management Agency.

"Having extra supplies on hand is always a good idea."

Wade says people should have a three-day supply of food and water.