I am opening my JPEG putting a shape on top and then creating a clipping mask. This works great and I end up with what I want, however my JPEG is still there in the background and the outlines still remain, meaning when I go to export it is exporting the size of the original JPEG as opposed to the clipping mask. if I hide the JPEG in the layers panel the colour in the clipping mask disappears.
That is all working as expected and is how things should work. Your image will always still be behind the clipping mask, thats how a clipping mask works—it masks the image, rather than destructively cropping it.
The problem is therefor how you are exporting your artwork. However you export your artwork there are basically two options, export everything or export your artboard[s].
Using Export, simply check "Use Artboards":
Using Save for Web, check "Clip to Artboard":
(Note, screenshots etc are from Illustrator CS6, things may look different in newer versions.)
If your artboard is not the same size as the artwork you want to export then select your image or clipping mask or whatever you want to export and go to Object → Artboards → Fit to Selected Art, which will fit the artboard to the selected object. You can then export at the correct size.
There is no way to change the size of an image in Illustrator that I know of, you would have to bring it into photoshop or lightroom or whatever program, make it the correct size or slightly larger, and then bring it into illustrator and apply a clipping mask. Illustrator retains the original file size so you can go back to the original size at any time which is usually quite helpful as people are fickle creatures.
If you'd like to reduce your file size, you can make your image a linked file while you're working on it and then embed it before exporting or sharing.
The best method I tried to resolve this is to create clipped image size art-board separately and copy each image to that and move to the asset export. Now click the small icon below that says "Launch the export for screens dialog".Select the art-board and rename it to the image name you want and click export art-board. Hope this helps!