How to edit dimensions for PCB layout in proteus
This can probably be explained better in a in a video but I'll try with images

Open ARES and place a similar footprint on the design (a relay that matches the number of pins is fine)



what you get is


right click on the footprint and select decompose tagged objects


If the dimension of your footprint are in mm then pres the m button for metric or leave it as is for mils


use the 2D graphics mode to make the outline of the component (or resize the existing one) to match the one you are trying to create


change the snap size if needed, then use the dimension mode and create lines or use the grid or numeric coordinates to help you place the pads (move the existing ones from the decomposed footprint) to the appropriate distances as per the device datasheet.


when you are done select the created footprint (pads &amp; outer line) plus the text from the decomposed component (this helps avoid entering everything from scratch), right click and select make package


in the next screen use a new name and change any other info you may want (not required)  and store the new footprint in a user library like USERPKG


In order to use the footprint in your actual PCB that is connected to a schematic you can double click the relay component and change the footprint name to match the new one you have designed, so this


is changed to


Another option is to change the linked package to the ISIS schematic editor.
Place a component that has the same number of pins (and names, or you have to do additional steps), right click and select packaging tool


press the add button to add a new package


select the package you have created


Set the footprint to schematic pin names association if necessary (not needed if names match) , set it as default (assuming you don't want to set it every time) and click assign package


then save it in a new library like USERDVC.

Note that after that you'll have two relays with the same name with one of them belonging to the USERDVC package, that is the one you should use


If you need a video tutorial then there are a few in youture, like https://www.youtube.com/watch?v=CHowCns-8IU