As a rule of thumb, when it is a Coat of Arms, resize the image to 600 pixel height

Make sure you lock width/height ratio

Make sure to disable transparant canvas

You want to maximize the map size, that still fits into a circle, that fits into an 800 x 800 canvas

Depending on the image, this can range from 680 to 800

Make sure you lock width/height ratio

Make sure to disable transparant canvas

As a rule of thumb, resize to a height of 350 or width of 350, whichever is the largest dimension

Make sure to leave transparant canvas enabled

Make sure you lock width/height ratio