I suspect you are referring to the file size (megabytes) rather than the image resolution/size (megapixels). The image resolution is determined by the number of pixels, which you can determine by multiplying the width in pixels by the height in pixels.
If you start with say a 10 MP image and don't crop is and don't have Aperture resize it (as you said, Full Size) then the result will be a 10 MP (megapixel) image.
But, the file size could be anything -- the Jpeg Quality setting will affect this -- for printing it should be pretty high, not just in the middle. Also affecting this is the amount of image detail as well as low light/high ISO noise -- when the software does a jpeg compression, a big expanse of clear bright blue sky will compress nicely down to a pretty small file (kilobytes or megabytes), whereas if you take a picture filled with leaves, rough wood, architectural detail, anything "busy", you will end up with a file that could be close to the original file size. Of course if you crop you cut down both the pixel resolution and the resulting compressed file size, and if you have Aperture resize, well that as well.
So, go over your Aperture setting while exporting an uncropped image with a lot of detail. Make sure Aperture does not resize the pixel dimensions (just don't resize) and make sure the jpeg Quality is at a high level, then export -- you should have a fair idea of what to expect -- if you had a lot of detail it should produce a good-size file (megabytes). Then do it with an image with a lot less detail and the quality and resolution will still be good but the file size will be significantly smaller.
Work through these things, it's not all that complicated.
Hope this helps!
