File size isn't really the issue here, although it's one of the factors. Your camera has a normal resolution of 5184 × 3456 pixels. It can produce JPEG files smaller than that, but the full size is the default. It also has various levels of JPEG compression, called "normal" and "fine". It will depend on the detail in your picture, but from this review I expect that images around 5mb means that you are already using the "fine" setting (and that this level is fairly aggressive on Canon cameras*). JPEG compression can steal away details, so you probably want to shoot in RAW.
It sounds like you're already doing test prints, and have concluded that:
When my image is to full size on the advertisement piece it is somewhat pixelated.
... which to me suggests that simply switching to RAW won't be the magic bullet here, because, stigma and superstition aside, JPEG compression isn't that big of a deal.
See Is there a general formula for image size vs. print size? about the pixel count you need in general, and What resolution would be needed for a billboard advertisement? about similar super-large prints in specific — for some uses, simply printing at the 54ppi you get from 5184 pixels spread across 96 inches will be fine. In many cases, your viewers might not ever be close enough to see the detail.
In other cases — like a poster advertisement that people see from far away and also might look at from right up close as they walk past — you may at least want the image to have non-jaggy look to the details. In that case, look at Is it possible to scale up an image to increase the the pixel density? and How can I upscale a low-res image to make it appear higher-res? for some approaches. You can't increase actual detail (sorry, "zoom and enhance" TV shows), but you can use various scaling algorithms designed to make the enlarged image look better (effectively hiding the pixelation).
* This is largely of academic interest, but there's more on in-camera JPEG quality levels at Is it worth using Pentax's Premium JPEG quality setting?