Hemingway said that "It's better to write about what you can write about and try to make it come off than have epoch making canvasses etc." Does this mean you should focus on what you can do instead of striving for groundbreaking achievements? Did he use the word "canvas" here as the one we know from painting? To refer to an artwork in general?
The comment seems to pose Hemingway as a miniaturist more interested in painting miniatures well than in undertaking a large-canvas work that might be beyond his current powers. Here is the paragraph containing the quotation, from Ernest Hemingway Selected Letters 1917–1961 (1981):
The Sun Also Rises could have been and should have been a better book—but first Don Stewart was taking a cure for his liver in Vichy while I wrote the first draft of S.A.R. instead—and secondly I figure it is better to write about what you can write about and try and make it come off than have epoch making canvasses etc.—and you figure what age the novelists had that wrote the really great novels.
Leonard J. Leff, Hemingway and His Conspirators: Hollywood, Scribners, and the Making of the American Celebrity Culture (1997) offers the following discussion about Hemingway's remark, which came in response to a letter from his editor noting the market success of The Sun Also Rises:
Max Perkins, also absent from the wedding, blessed it with sales reports on The Sun Also Rises. "We get small orders from points all over," he wrote several weeks before, noting that the novel was at nineteen thousand copies. The reports were a subtle reminder, as Hemingway collected stories for Men Without Women, that a novel was necessary for the advancement of his career. Hemingway shrugged off the hint. It was "better to write about what you can write about and try and make it come off than have epoch making canvasses," he told Perkins. The declaration may have persuaded author more than editor.
In the wake of the massive novels A Farewell to Arms and For Whom the Bell Tolls, it may be difficult to think of Hemingway as the literary equivalent of a miniaturist, but that appears to be how he was describing himself to his editor on November 19, 1926.