What is Angular Diameter

In this image you can find the actual scaled size of Mercury in front of Sun disk. That is marked by Red line. P.C. Wikipedia

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

In general perception the tip of your little finger will create 1° by holding the hand at right angles to a fully extended arm. And then the size of Sun disk is exactly the half of your finger tip. So you can well imagine how small the Sun appears in our sky. The size of sun then is ~31° means 31 X 60 = 1860" (Arcsecond). The size of Mercury on event day will be 12" so that will appear as  1/300th times of your little finger tip. So you can well imagine the size. For reference look at the picture. If the red line will be the observable size of Sun (By the help of Optical Instruments) then you can find mercury as a dot (.) appearing in the Sun disk.