It's true that A Suitable Boy is very long, but it's also true that Seth then deliberately set out to write a much shorter work for his next book (partly) because: "I also reckon that publishers aren't intrinsically fond of long novels; they're difficult to convince people to read...or to review for that matter." In addition, he was a published poet - a field even harder to break into than novel-writing - before he became a novelist.
His experience shows that it can be done, but for every one book of extraordinary length published by a debut author there are many, many more published that are average length. Only the individual author can decide if his or her vision of a very long novel is worth the added difficulties in getting published.