Media Settings.
When these are filled in the files can be resized during upload. The bold values below are my recommended sizes.

Pages can be very slow loading if images are too large in file sizes.
640 pixels width would fill 3/4 of a moderate width screen.
Untick the auto crop of thumbnails, your thumbnails should be same proportion as your larger image either portrait, landscape or both.
The sizes are same in width and height, as these settings are Maximum . . . a landscape 640 x 480 pixel image would upload as exactly that, or a portrait 480 width 640 height, retaining their natural proportions.

The sizes listed below determine the maximum dimensions in pixels to use when inserting an image into the body of a post.

Thumbnail size
Width 150 Height 150
UNTICK BOX Crop thumbnail to exact dimensions (normally thumbnails are proportional)

Medium size
Medium size Max Width 300 Max Height 300

Large size
Large size Max Width 640 Max Height 640

Embeds
Auto-embeds -Attempt to automatically embed all plain text URLs

Maximum embed size Width 400 Height 400

Uploading Files
Store uploads in this folder Default is /wp-content/uploads

OR Full URL path to files

Default setting for images wp-content/uploads is fine, leave the setting as is.