The ruff is the costume
The defining feature of a Maine Coon in painting is the chest ruff — a thick collar of longer fur that frames the face. In Royalty portraits this reads as natural court regalia: the ruff IS the high-collared robe. We hold the ruff at the volume it has in your photo, dense or thin, summer-trimmed or full winter.
The ears, the tail, the snowshoe paws
Three secondary features we preserve carefully: the lynx tips (extra pointed fur at the ear tips), the long bushy tail usually carried high, and the snowshoe paws with their tufted fur between the pads. Together these are what make a Maine Coon visually different from any other long-haired cat. Lose them and the portrait could be any Persian.
What the size lets you do
Because Maine Coons read as substantial in painting, they carry larger formats well. A 60×80cm Canvas of a Maine Coon as King or Knight has the weight a smaller cat couldn't pull off. For watercolor or Cherry Blossom scenes, the size becomes the contrast — a massive cat among delicate petals reads as intentional, not awkward.