Do certain subjects require certain sizes of biography? This question arose while reading a review of Peter Parker’s massive biography of Christopher Isherwood (Random House, 832 pages, $39.95). Can an 800-page biography of Isherwood be warranted, one reviewer asked? A James Joyce or a Henry James, perhaps, justifies such length, but a minor novelist?

It is the wrong question, but reviewers often ask it because they are besotted with an idea of literature that has warped the reception of biography. Biographies per se have nothing to do with establishing literary value. They are the story of a life – anyone’s life. Of course, a biographer can argue for the literary value of his subject’s creations, and the biography itself may aspire to be a form of literature, but to apply some standard of “greatness” to measure a biography before you have read it is to destroy the idea of biography itself.

I sense in that reviewer’s question a condescension toward the genre: one that appears in some form almost every week in the press. For many reviewers, biography seems a second-rate genre, beholden to its subject matter. So allot Isherwood, what, 300 pages? Does that signify minor? What then of a T. S. Eliot biography that is 200 pages? Is it insufficiently committed to its subject?

The answer, of course, is that a biography is as long as it needs to be to suit the biographer’s objective. The question to ask about Peter Parker’s book is whether its length is suited to its purpose. He has attempted to produce a definitive life, and, though there is no such thing in biography, he has come as close to it as anyone writing now can hope to.

There are biographies you can just wallow in. They navigate deftly through waves of detail that give the book a buoyant feeling. Biography of this kind cradles you in the swell of a life. It is pointless to say that Mr. Parker’s work contains too much, Isherwood’s life lends itself to this encompassing approach. He is an engaging biographical subject – by which I do not mean exceptionally dramatic or brilliant. Rather, he is attractive because of his need to share himself with others – a kind of fundamental generosity that forges links in so many different directions that he becomes a focal point and an idol.

Isherwood was a pathfinder. Slightly older than his friends W. H. Auden and Stephen Spender, it was Isherwood, Mr. Parker shows in fascinating detail, who was the leader. Certainly Auden surpassed Isherwood as a writer, but at the beginning and hub of it all stood Isherwood, rejecting old England and the privileges of class, living an openly gay life before anyone thought it was gay, and promoting, as Mr. Parker puts it, ” a character of his own devising.”

Isherwood emerges as a heroic figure who invented a new way to be a literary figure and citizen of the world – not an inconsiderable achievement for a well bred boy whose mother wanted him to remain at home forever, dedicated to the memory of a sainted father who died valiantly in World War I. The thought of such sacrifice terrified Isherwood, and Mr. Parker does not blink at his subject’s cowardly side – or at the egomania that went hand-in-hand with his rejection of traditional British values.

This is biography on a magnificent scale, one true to the genre, which is the unfolding – not the measurement – of a life. Thus Isherwood’s highest achievements – such as the Berlin stories that enshrine him not only in literature but also in the musical theater pantheon – are not employed in the fashion of literary critics to dog the subject. Instead, each new project Isherwood embarks on is shown to be fraught with promise. This is biography as “Great Expectations.”

Mr. Parker is devoted to his genre and so must point out how often Isherwood exaggerated in his memoirs or simply made things up in service of a good story. The biographer is often chided by reviewers for such truth telling, for the small-minded bookkeeper mentality. Do not trouble us with so many details, say the critics! But Isherwood was a great collector of data, which he then crunched into inimitable narratives. Mr. Parker’s greatest virtue is to show how Isherwood did the crunching.

A great character emerges in the process. The mother Isherwood vilified turns out to be a far more complex and sympathetic character than her son could ever acknowledge if he was to maintain his stance as a modern man unconstrained by convention. She tolerated her son’s lectures, often gave him the money that made his escapades possible, and never forgot – as her son tried to do – how close they once had been, when Isherwood, the boy, mourned and celebrated the tragic death of his heroic father.

There is a side to Isherwood the runaway boy that again reminds me of Philip Pirrup. Like Pip, Isherwood had his pretensions and wanted to jettison a background he thought unworthy of his high dreams. But the biographer, like Abel Magwitch, returns the protagonist to his origins, linking past and present, self and surroundings, just as all outstanding biographers and novelists are wont to do.