You don't need to do that.
#1 Just drag and drop OptionInfo.lua from the folder titled "savedata" onto notepad and edit the resolution to your liking, save it and run the patcher again. The new client should have your custom resolution.
#2 Note that doing so will reset your default resolution to your custom. Also, whichever client you close first, will have their resolution autosaved into OptionInfo.lua. This means that if you closed the smaller client, all new clients from then on, will open with a smaller resolution. You can use the first step to revert all clients back to your original, provided there are no small clients active. Or you can revert back by making sure the last client to be closed has your original resolution.
I mention this because, in the event of a disconnection etc, assuming your current client was a custom small resolution, when you change it back to your original, you will now have to drag all hotkeys, chatbars etc back to their place because the resolution change will make their position out of whack.