One hundred pixels sounds modest. But imagine a star the size of our Sun, 4.3 light-years away. Its roughly 1.4-million-kilometre diameter would occupy only about 0.007 arcseconds of sky.
For an image with 100 pixels across the star’s disk, with roughly two pixels per resolvable detail, the ideal telescope diameter is about 1 kilometre. For 100 separately resolvable detail scales across the disk, it is about 2 kilometres.
These estimates use green visible light at a wavelength of 550 nanometres and the standard Rayleigh diffraction limit. They assume excellent optics and negligible atmospheric blur. Actually detecting surface features also depends on their contrast and the quality of the measurements.
The calculation starts with the star’s angular diameter in radians: its physical diameter divided by its distance. Dividing that angle into 50 or 100 detail scales gives the required resolution. For a circular aperture, the Rayleigh limit is 1.22 times the wavelength divided by the telescope diameter. Rearranging gives about 980 metres for 50 detail scales, or 1,960 metres for 100.
The interesting catch: pixel count alone does not require a huge telescope. You can enlarge a blurry spot to any size. The kilometre-scale requirement comes from wanting those pixels to contain detail from the star’s surface. Two pixels per detail is an approximate sampling choice, not a promise of a particular number of visible features.
A tiny picture of another sun could demand an enormous telescope.