It's not just about pixel sizes though, but also about the inherent sharpness of the image. To exaggerate, if the image had tons of mpx but the camera had been handheld at 1 second (ignoring any attempts at icm), you might want to downsize it ...
That is why I said
if a 10x8 looks good in the hand.
An image can still look good if it is soft , even unsharp, and of low pixel count with very little detail, provided that the tonality is good and the image is interesting.
Image quality, and what makes an interesting image, is extremely hard to define. and what looks good, even outstanding. need follow no rules in the conventional sense.
In the normal way a "Good" Or "Excellent" image will look equally good at any size.
The Digital age has created a preoccupation with detail and sharpness, quite out of step with more important image qualities. like Subject treatment, Artistic merit, lighting or intrinsic interest.
Modern high pixel counts and high resolution lenses emphasis the extraordinary shallow depth of field obtained at any aperture when an image is viewed closely.
And demonstrate the extreme difficulty of preventing loss of ultimate sharpness, due to camera movement, even at high shutter speeds.
When counts reach the level of 100 megapixels. these difficulties are almost overwhelming, except perhaps in the most extreme studio conditions.
Hand held results with such equipment, almost invariably, show lower definition than the sensor/lens is capable of. (Albeit, probably, still outstanding by any other standard)