In the current release of mojoPortal (18.104.22.168), it is not possible to make the image gallery use full size images in Nivo. But, I just now implemented a display setting to enable using the full size images, so it will be possible in the next release. You can get this change now if you are able to make your own build from our repository, otherwise it will be in the next release. You will be able to set the new display setting NivoUseFullSizeImages="true" from the theme.skin file.
Only suggestion for now, if you still have the original images, delete them from the gallery, make the web size settings bigger in settings and then upload them again to re-create the web images at larger size.
Hope that helps,