I do not think you are going to find a 'trick' to accomplish this in a simple set of commands. Writing a for loop to adjust the images of each actor object and their location within the world with respect to the ratio of the change in the world size would seem to be the only way to go.

Reponse to previous answer of Dan post ( No :-) , the first world is a menu and the second is a map where my characters are. (menu just have a button on it and when I click on it, it set the second world). I want to change the size of the second world but keep the characters which are in it at the same place and change their image to have exactly the same result at the end but smaller. I want that because when I take my project and put it on a .jar file the size of the windows of my game is 1024x768 and don't change when I set the second world. ) Edit : In fact the size of the windows is not 1024x768! I don't know which size it is. :/ Is there any method to just change the size of the windows and don't change anything of the game ?

The 'super' command (first line) in the constructor of the world determines the size of the world. I do not see how you could not know the actual size of the world as this is what it will be in the jar file also.

Of the three button on the top-right of the Greenfoot window frame, you have (1) minimize (2) full-screen (3) close buttons; click on the full-screen button. Hide whatever you can that is taking away space from the application (taskbars, titlebars, etc.). Also, reducing the length of you class names and the depth of sub-classing will aid in adding more horizontal space to the main window.

Haha ok I thought there was a method for that, sorry. Hum ok I'll try

Sorry to hear that. If you have maxed out the scenario window size and it is still too small, I do not know what else you can do. BTW, the largest I could make mine is (1200, 640)