Hi Peter,
There is server configurations that control how large a file can be uploaded. I'm pretty sure that setting would also limit uploads by the aggregate size of all files that are uploaded in bulk at one time.
See Supporting Large File Uploads
The Image Gallery has 3 different slide show options, 2 javascript based and the one silverlight one. If you can't use Nivo, there is still one remaining option. With Nivo not checked, check the box for Use Slide Show and check the box for Start Slideshow Automatically. It uses a colorbox which can be styled by css.
The nivo orman theme no longer works because it was from an older version of Nivo. We upgraded to the latest version of Nivo and it has no orman theme but has a few others. I recommend use one of the other themes.
Also I think the same size requirement was only for the older version of Nivo, you should be able to use any size with the latest version as far as I know (subject to the resizing done to create web size and thumbnail size from the uploaded full size iamge based on settings). In the latest version of mojoPortal we link the web size image to the full size one.
Hope that helps,
Joe