Background image resize in Ubuntu 18.04.1 LTS

to know the current picture-option run the below command in terminal
gsettings get org.gnome.desktop.background picture-options

Example:

pratap@pratap:~$ gsettings get org.gnome.desktop.background picture-options
'zoom'

in my case, the result is 'zoom'

to know the available options for 'picture-options' run the below command in terminal
gsettings range org.gnome.desktop.background picture-options

Example:

pratap@pratap:~$ gsettings range org.gnome.desktop.background picture-options
enum
'none'
'wallpaper'
'centered'
'scaled'
'stretched'
'zoom'
'spanned'
pratap@pratap:~$ 

to use the option centered from the available options from above range, use the below command
gsettings set org.gnome.desktop.background picture-options 'centered'

Example:

pratap@pratap:~$ gsettings set org.gnome.desktop.background picture-options 'centered'
pratap@pratap:~$ 

Use as below

for Background = org.gnome.desktop.background
for Lock Screen = org.gnome.desktop.screensaver

GUI way

open gnome-tweaks.
if not yet installed, install it by the command sudo apt install gnome-tweaks

find the options under Background and Lock Screen as Adjustment and choose from available.

background color can be changed with this answer as a guideline only. https://askubuntu.com/a/1090204/739431 available options are solid, gradient horizontal and gradient vertical.