When you upload images to your workspace, they are saved at original size, but are scaled to fit in the wiki page when displayed.

To change the displayed image size, open the page for editing in Advanced mode. Click on the image, and choose the size you would like.

If you are using Safari and cannot use Advanced editing mode, you can still resize images. Open the page for editing and find your image.

Add "size=small,medium,large" or "size=X" (where X is the width in pixels you would like).