plotting – Scaled Axis when drawing circle

Use the option AspectRatio -> Automatic:

Plot[{}, {x, 0, 1}, 
 Epilog -> {Red, Circle[{.5, .5}, .5], Red, 
   Line[{{0, .993}, {.5, .993}}], Green, 
   Line[{{.5, .993}, {.5, .5}}]}, 
 AspectRatio -> Automatic]

enter image description here

More straightforward way to get the same result is to use Graphics:

Graphics[{Red, Circle[{.5, .5}, .5], 
  Red, Line[{{0, .993}, {.5, .993}}],
  Green, Line[{{.5, .993}, {.5, .5}}]}, 
 Axes -> True]

enter image description here