You must design the icons in three sizes, 24x24, 32x32 and 48x48. The first one is to be stored under the name of the component as before, the middle one has an appended _150 (for 150%), the last one _200 (200%), e.g. tpanel.png, tpanel_150.png, tpanel_200.png.
As an example, just look at folder (lazarus)/images/components. The script which creates the res file for the IDE is (lazarus)/images/components_images.bat, the needed file list is components_images_list.txt