As per the silent image spell description

You can use your action to cause the image to move to any spot within range. As the image changes location, you can alter its appearance so that its movements appear natural for the image. For example, if you create an image of a creature and move it, you can alter the image so that it appears to be walking.

Say I make a fog or darkness effect using Silent Image that is 10ft by 10ft when I subsequently move the spell as an action can the size be changed? ie 15ft by 15ft or 5ft by 5ft.

The description of the silent image states:

You can use your action to cause the image to move to any spot within range. As the image changes location, you can alter its appearance so that its movements appear natural for the image. For example, if you create an image of a creature and move it, you can alter the image so that it appears to be walking.

Say I make a fog or darkness effect using silent image that is 10 feet by 10 feet. When I subsequently move the spell as an action, can the size be changed (i.e. 15 feet by 15 feet, or 5 feet by 5 feet)?

As per the silent image spell description

You can use your action to cause the image to move to any spot within range. As the image changes location, you can alter its appearance so that its movements appear natural for the image. For example, if you create an image of a creature and move it, you can alter the image so that it appears to be walking.

Say I make a fog or darkness effect using Silent Image that is 10ft by 10ft when I subsequently move the spell as an action can the size be changed? ie 15ft by 15ft or 5ft by 5ft

As per the silent image spell description

You can use your action to cause the image to move to any spot within range. As the image changes location, you can alter its appearance so that its movements appear natural for the image. For example, if you create an image of a creature and move it, you can alter the image so that it appears to be walking.

Say I make a fog or darkness effect using Silent Image that is 10ft by 10ft when I subsequently move the spell as an action can the size be changed? ie 15ft by 15ft or 5ft by 5ft.

As per spell description

You can use your action to cause the image to move to any spot within range. As the image changes location, you can alter its appearance so that its movements appear natural for the image. For example, if you create an image of a creature and move it, you can alter the image so that it appears to be walking.

Say I make a fog or darkness effect using Silent Image that is 10ft by 10ft when I subsequently move the spell as an action can the size be changed? ie 15ft by 15ft or 5ft by 5ft

As per the silent image spell description

You can use your action to cause the image to move to any spot within range. As the image changes location, you can alter its appearance so that its movements appear natural for the image. For example, if you create an image of a creature and move it, you can alter the image so that it appears to be walking.

Say I make a fog or darkness effect using Silent Image that is 10ft by 10ft when I subsequently move the spell as an action can the size be changed? ie 15ft by 15ft or 5ft by 5ft