My personal headcanon is a large man around mid-40s, athlete when young but less so, tall and heavily built. That's one of my favorite types, so why not? Some writers describe him as "perfect" but that's not what I imagine. Wodehouse describes him in stage directions for an unproduced play as being in his 40s. I'll try to look that up at some time. In "Ring For Jeeves", the only book without Bertie, he is described, among other things, as looking like the youngish High Priest of a dignified religion.