To know how large each celestial object will appear in the sky at first you need to have an idea of Angular Diameter.
The angular diameter or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view.
In astronomy the sizes of objects in the sky are often given in terms of their angular diameter as seen from Earth, rather than their actual sizes. Since these angular diameters are typically small, it is common to present them in arcseconds. An arcsecond is 1/3600th of one degree.
Degrees, therefore, are subdivided as follows:
360 degrees (°) in a full circle
60 arc-minutes (′) in one degree
60 arc-seconds (″) in one arc-minute