Include the parameter size and the dimensions in the format of width x height (no spaces) e.g. 300x200 as the value. Just the width is also ok, and will produce a square.

Examples:
https://mkto.jayjiang.com.au/tools/placeholder/?size=150
https://mkto.jayjiang.com.au/tools/placeholder/?size=600x200

Additional Options:

Include the parameter bg and a hex color code for different background colors.
Example:
https://mkto.jayjiang.com.au/tools/placeholder/?size=150&bg=a3ffa5

Include the parameter fg and a hex color code for different text colors.
Example:
https://mkto.jayjiang.com.au/tools/placeholder/?size=150&bg=a3ffa5&fg=0000ff

Include the parameter text and an url safe string for different text.
Example:
https://mkto.jayjiang.com.au/tools/placeholder/?size=320x180&bg=a3ffa5&fg=0000ff&text=16:9%20ratio