FlipTheScale

Every instrument has a smallest detail it can separate, its resolution. Below that, two points blur into one. For each thing, pick the simplest instrument that can still resolve it.

Honest notes

Each limit is a best case. A real picture is often blurrier: a stained cell in a light microscope, or a frozen sample in an electron microscope, rarely reaches the limit.
Seeing something is not the same as resolving it. You can spot a star or a single hair; you cannot resolve its width.
Sizes are typical and rounded; real things vary.

Sources

Sizes: the TenTimes ladder (Milo and Phillips, Cell Biology by the Numbers, and textbook values); a hair: Ley 1999, The Physics Factbook; an α-helix turn: Pauling, Corey and Branson 1951.
The eye: James Madison University, Resolution and Size Primer.
The light microscope: Nikon MicroscopyU, Resolution.
The electron microscope with biological samples: Faas FGA et al. (2012). Virtual nanoscopy. J Cell Biol 198:457–469. doi:10.1083/jcb.201201140
Atomic models: Nakane T et al. (2020). Single-particle cryo-EM at atomic resolution. Nature 587:152–156. doi:10.1038/s41586-020-2829-0