L.A. Noire will be 8sq.Miles of area for you to explore. From edge to edge it would take you 30minutes to traverse in a car.
To put it in perspective here are a few game worlds you might be familiar with.
Mafia II - 4sq.Miles
GTAIV - 6sq.Miles
The Elder Scrolls III: Morrowind - 10sq.Miles
The Elder Scrolls IV: Oblivion - 16sq.Miles
Just Cause 2 - 385sq.Miles
FUEL - 5560sq.Miles