I created a board outline for my pcb, but when I generate the Gerber plot, the outline is not correct. The left side passes through the pcb, and it is not the correct size.
Very strange! Have you checked the settings for all layers under Plotting & Printing=> Position?
How are you viewing your Gerbers? Try Ucamco.com. If you are using a third party "simulated pcb viewer" (e.g. from Eurocircuits) they often get the pcb outline incorrect.
Try adding a (top & bottom) silk screen pcb outline (superimposed on the pcb outline) to see how that compares.
Thiis is my personal opinion, but over the years I've been assisting with Gerber plots, the viewer which has had most instances in incorrect display is GerbV. They're usually pretty good about fixing these instances, but it's odd that this viewer seems so much more prone to errors than others.
A caveat. If you're looking for alternative viewers, it's best to trawl forums to find which are highly regarded. A small minority are fake, intended to plant trojans, so please be careful and research any new ones!