Recumbent readers beware. Those who like to do their poring lying down will scarcely rush to take up this book. It is monumental. Once hefted, however, and well balanced on lap, knee or chest, ''Citizens'' will prove hard to put down. Provocative and stylish, Simon Schama's account of the first few years of the great Revolution in France, and of the decades that led up to it, is thoughtful, informed and profoundly revisionist. Mr. Schama, who teaches history at Harvard University, has committed other large and readable tomes. But nowhere more than here does he challenge enduring prejudices with prejudices of his own. His arguments, though, are embedded in narrative. Above all, he tells a story, and he tells it well.