I need to run a graphical program on i3 window manager but it's automatically sized then put on the split screen when I launch it :
$ source my_gui
There is no way to start an application as floating, but you can create a rule which to tell i3 it should be in floating mode. You can create rules based on x properties like class. If your window has "MyApp" as WM_CLASS you can use
for_window [class="MyApp"] floating enable. Of course you can also use different properties.